OK - removed the rear shocker mounts and after copious doses of the precision adjustment tool it seems like the rot is confined to the area just in front of the shocker mounts on the top of the chassis and a little bit at the side.
The really good news (I think

) is that shining a torch in to the chassis through the holes has revealed what looks to be a very good inside with very little evidence of rust at all. That being the case I think I'm going to get it plated with 3mm steel in a full section running about 6 inches either side of the shock mount and also in the top of the chassis rail.
Couple of questions:
1. The big heavy crossmember just in front of the shocker mounts - what's it called and can you buy them already fabricated to weld on? (there are some holes in this and it looks like it could do with replacing - it'll also provide better access to weld the top of the chassis rail).
2. Dave W (primarily) - you mentioned in an earlier post the bracket that holds the fuel tank on to the chassis (radius arm?) - is this something that should be permenantly fixed to the chassis (welded) and then bolts on to the fuel tank? Can this be bought separately and what is it's proper name.
Thanks for the comments on the galv chassis - I could easily see myself forking out £1,200 for the chassis plus another grand for all the other bits I'd feel compelled to replace as I rebuit it. It's not an everyday use vehicle, just going to use it for trialling really so can't justify spending loads on it.
Keep the comments/opinions coming.
Cheers
Mark