The cases are back together. I used Threebond 1215 liquid gasket which is the recommended stuff to use.
It's now waiting for all its bits and pieces to be reattached.
The pistons looked OK, but quite worn, and there was some evidence of blow-by.
I've no idea how long they've been in there, so I've gone with the recommendation of the guys on the RGV forum and will be rebuilding with new pistons. Gives me the maximum length of time before I need to do anything again and means I can wring its neck all next summer without worrying too much.
While I'm waiting for lots of new parts to arrive, I gave the frame another good looking over and spotted this on the top shock mount that I hadn't spotted earlier.
Here's a closer pic of what looks like the beginnings of a crack. I've sanded it down a bit to make sure it wasn't just a surface defect.
I found these pics posted by a member of the RGV forum and it looks like I've possibly got the beginnings of a similar failure.
http://s1212.photobucket.com/albums/cc447/beef_99bikes/
That could result in some interesting handling characteristics, so I'll take a cautious approach and fit a new piece. It's a removable bracket and not part of the frame so easy enough just to swap it (although I expect those bolts will be a tw*t to get out).