Sorted it this afternoon! I ended up removing the pillion grab rails, then the plastic silver rear frame surround which the seat locking mechanism is fixed in to. Took that indoors, turned it upsisde down and could see the seat locking mechanism. It has a cover (item 9 in the part diagram I posted) so I took that off and as there are some spirngs around the spindle that turns when you ture the key, and the front seat arms, it all sprung out. With some help I managed to get it all back in place and replace the cover and hey presto, it worked. I couldn't see any reaon why it wouldn't work before I took the cover off, except that it wouldn't. Reseating it all and putting the cover back on seemed to do the trick. I've put it all back on the bike and it's all working so fingers crossed it stays that way.
Many, many thanks to all who posted here, especially for the photographs. Now to get out and ride! :-D