Thanks.
At the moment everything is working, I'll leave well alone.

Thanks Tim, it's working again! :)
The final solution was to start again from scratch, and reinstall v234, whereupon everything worked.

I don't really know what caused the initial trouble but suspect missing ports in the firewall open lists on one or both PCs, as this is a bit of a jungle to me. From the great list of firewall rules incoming and outgoing it looks as if many programs just request to open "any" so I got started by doing the same for MSFS and Plan-G, then narrowing it down to what still worked. 

Also, possibly a subject for another topic: in going over everything I found a remarkable number of copies of SimConnect.dll on my C: drive, about 50 in all!
That included copies in three versions of the SDK, some with name prefixes like BeatlesBlog.SimConnect.dll, Lockheed.Martin.Prepar3D.SimConnect.dll, Microsoft.Flight.Simulator.SimConnect.dll, some in folders associated with FSX and FSXSE, and also leftovers from older Plan-G versions and folders used in unzipping etc., and some in the recycle bin and temp folders

I got rid of the obvious ones, but am left with 18 copies, for FSXSE and MSFS, and their SDKs.
Which begs the question - which simulators and addons require which versions? Are all these versions still required, or if not, is last-dated always best?   

I've made this a new topic - I do not have FSUIPC and cannot connect using any version of SimConnect.

Background:

Several weeks ago I had v4 build 228 working locally and remotely with SimConnect on the original release of MSFS
I stopped using MSFS for several weeks for unrelated reasons. Recently I came back and applied all the updates to MSFS. Am now running the latest, 1.11.6.0
The sim itself is working OK even with many remaining bugs.

Tried Plan-G v4 build 228 again and it didn't work. There was just no connection (not surprising I suppose).

After updating myself on this very busy forum, I read the "Quick Start" and installed build 234, on both local and remote computers.
It worked straight away on the local computer, built a new database, with no serious problems and plenty of improvements, thanks Tim.

Copied and pasted the new data files to the remote computer.
Checked that nothing on the two computers had changed, such as IP address, port nos, firewall ports open. Made some minor changes to the .xml and .cfg files, like layout and increase to MaxReceiveSize, now 41088.
Checked that SimConnect.dll from the new SDK is available on the local computer. Is this "Simconnect2", and does it also need to be available to the remote computer? If so, which version(s)?

v234 ran OK on the remote computer except there was no "connect" -  no aircraft icon on the map.
Turned firewalls off and on several times, no difference. Tried three methods of connection, SimConnect1, SimConnect2, BeatlesBlog.
Sometimes the connect simply timed out, with a chime and return to the Home screen. Sometimes it crashed to the desktop.
So, sadly, no success.
Copies of SimConnect.cfg, SimConnect.xml, and Plan-Gv4.log are attached

I hope it's something simple and that someone can put me right please.

No, apart from the obvious (obvious now, that is <g>) -- there must be a SimConnect.cfg and there was none.

Other possibilities -

      (1) mix up re. 32- and 64-bit versions of Dotnet -- when I originally installed 228 I assumed 64-bit but it wouldn't run, so went back and installed 32-bit, which worked. I left both versions installed. Irrelevant maybe.
      (2) need for a bigger receive buffer - the clue here is in the MS Simconnect SDK stuff, they use 41088 so I followed suit. Someone better qualified might be able to throw light here.

For the record, my SimConnect.cfg ... (address and port to suit your own installation of course).

  [SimConnect]
Protocol=IPV4
Address=192.168.0.5
Port=4504
MaxReceiveSize=41088
DisableNagle=0

Good news here too Tim, v4 build 228 working fine on remote laptop under SimConnect.
In my case the breakthrough was putting a SimConnect.cfg file a la Prepar-3D in the Plan-G program folder. That caused a bit of head-scratching, and might be the cause of problems by some.

Some praise where it is due -- for another very nice new feature - the Bing maps feature in Plan-G maps. There was little wrong with the old maps, but this takes things to a new level - now we can fly to the Bing satellite scenery in pretty good 3D near-photo quality, while tracking in Plan-G on street maps that correlate exactly. Very nice, thanks.

Missed before - you don't need ipv4, use pipe.

I had my FSX SE on Win10 Pro working happily with Plan-G 3.2.1.151 under FSUIPC before using the remote (SimConnect) version.
What problems have you had under FSUIPC ? It's unusual for it not to work, and relatively easy to debug. SimConnect on the other hand can be a nightmare, and should not be necessary when Plan-G is on the same machine.

Things that come to mind though (if you really want to persist with SimConnect) are:

1.) The SDK should be present as a folder named SDK under Program Files (x86) > Steam > steamapps > common > FSX. This would have been installed at the time FSX was installed by Steam I think.

2.) There must be two files, named SimConnect.xml and SimConnect.dll in the same folder as your scenery.cfg and fsx.exe. In Steam Edition that would normally be in  the same folder as 1.)  My .dll file has the version id of 10.0.62615.0 (FSX 20150710-1055) with copyright by Microsoft. According to the manual. you don't need to do anything to either of these files.

3.) Unless I'm mistaken, your setup is covered in some detail in section 18 of the Plan-G manual, and there is more in section 20.  Just to be sure though, I think you could check the SimConnect .xml as per section 20, remembering that we now have a new version of Plan-G plus the Steam edition of FSX which is also a little "different".

4.) Look for a SimConnect.cfg file in the folder  "This PC > Documents > ". This worked wonders for me, but I have no idea of its relevance, nor why it would be needed when Plan-G is on the same machine as FSX. It's not mentioned in the manual, but this is what mine looks like:

*******************************************
[SimConnect]
Protocol=Auto
Address=[mine has the FSX machine's ip address]
Port=[mine has the FSX machine's port number]
MaxReceiveSize=8192
DisableNagle=0
*************************************************

End of sermon ... Good luck!

OK, after another day spent reading and debugging, I've got mine working. There have been four things done since yesterday.

First, to recap, I am running FSX SE on a just-upgraded i7 desktop with Gen 8 Intel processor, Win 10 Pro all activated, updated and proper. It is networked via wired LAN and wireless to a laptop also running Win 10 Pro and Plan-G v3.2.1.151.  The previous setup which worked perfectly was identical except for the upgraded hardware, and the desktop ran Win 10 Home.

The four things are:

1.) Installed the SimConnect config tool on the desktop, and ran it. This itself was a jungle of confusion, and anyone wanting more information has only to ask while it's in my head. It simply automates the process outlined elsewhere for setting up the SimConnect system on the FSX machine.

2.) Reinstalled the FSX SDK on the laptop. It goes in a subfolder under the Program Files (x86)/Microsoft Games folder and is necessary for some reason.

3.) Used my Microsoft account (not the usual network or local account) to log on to the desktop from the laptop. Suddenly the Plan-G "locations" worked. I suspect this is part of the new security paranoia surrounding the Gen 8 chips, somebody might know more?

4.) Most significant: created and installed a SimConnect.cfg  file on the laptop, under "Documents". See the SDK itself for details. Interestingly, it was not there before but the old system worked, so it might be a new requirement under my setup.

The Navigation Database that I had already installed was accepted OK, and I can refresh it now.

At last it all works, and all the frustrations of the last few weeks can be put behind. On with the flying!   

Same here, but with the added joy that I am now using FSX SE on a desktop Win10  and a Gen 8 i7 (coffee lake) and a B360M chipset motherboard - i.e. the new "militant" Win 10 combination with horrendous security and permissions barriers, which I'm discovering every day.

I wish I'd saved my money, the old 2nd-gen i7 was pretty good in retrospect.

Everything worked perfectly on the old Win 10 setup, including SimConnect, but nothing seems to make sense now.

Have you created a new SimConnect.xml file as per the Plan-G manual, sections 17, 18, 19?

I've done everything according to the book, but the copy of Plan-G v.3.2.1.151 on a laptop (on which I have been running Plan-G) can't connect to or even see the Steam folder on the networked desktop, let alone generate a navigation database.

In Windows Explorer I can map the Steam folder easily, and all permissions are in order (I think), because I can do anything in Win Explorer. The "browse" buttons in Plan-G file>options>locations just do not see the remote machine.

I should add that the sim itself is working perfectly, and new Plan-G also works perfectly from the same pc using FSUIPC. I have also tried copying across the "data" folder to circumvent creating a new database, but to no avail.  Plan-G just accepts the datbase ok, but hangs and terminates itself after about 4 seconds when I click "connect".

Is anyone else having similar troubles? What about the WideFS method?