Okay folks. The RSGB GM replied and as a result I have had a lengthy phone conversation with him - nice bloke unlike the one who left under mysterious circumstances. He has clarified that membership of an affiliated organisation does not automatically confer cover on members of that organisation in respect of their own activities - only those activities that are explicitly organised by the affiliated organisation. He will seek to have this clarified on the website.
Furthermore he has said that he will raise the issue of liability insurance for personal portable operating with their insurers and then take a view on whether any likely cost would be offset by increased membership. I pointed out the cycling model as both British Cycling and the group for pensioners, the CTC, offer this insurance as part of their membership package. In the case of cycling, having such insurance is often a requirement to join a cycling club. The potential risks involved in cycling are very different from what we do of course.