Many thanks, Tim, that's definitely solved it.

It looks like an up-to-date file, Tim, and Bournem'th is abbreviated in the VRP name.
Just to be sure I've deleted everything bar the .zip file and re-installed the program.  It's still the same though.
This is the .pln:
[flightplan]
AppVersion=9.0.30612
title=EGHH to EGHJ
description=EGHH, EGHJ
type=VFR
routetype=0
cruising_altitude=1700
departure_id=EGHH, N50* 46.65', W1* 51.42', +000036.00
departure_position=8
destination_id=EGHJ, N50* 40.68', W1* 6.55', +000052.00
departure_name=Bournemouth
destination_name=Bembridge
waypoint.0=, EGHH, , EGHH, A, N50* 46.65', W1* 51.42', +000036.00,
waypoint.1=, BOURNEM'TH, , BOURNEM'TH, I, N50* 42.43', W1* 44.56', +000000.00,
waypoint.2=, EGHJ, , EGHJ, A, N50* 40.68', W1* 6.55', +000052.00,

Edit: for information - just noticed same problem with another flightplan that uses Bournem'th Stoney Cross

In version 9, an error occurs on opening a flightplan containing this waypoint.  The error message is:
"Plan-G is unable to open the file you have selected. It may have been moved or deleted".
Version 8.1.389 opens the file correctly.
To verify, I created a simple plan Bournemouth to Bembridge direct; this was fine but after adding the VRP and saving the plan, it couldn't then be opened.