OK..... the site is now up and running again...
Here's roughly what I did to get it working... painful.. but..
Setup another copy of the new software on my test server and installed the demo data.
Once that was working, blew away the database and uploaded my data into the database instead... all continued to work.
Added in my template and managed to get it working with that without too many hassles (turned out some of the images were missing, so sorted that).
Then started adding my customisations back in... one by one, using a diff tool to look at the files and directories that were different from the fresh install to the partially working one.
Found that I had something (still not sure what) wrong with the overide for the details page (pretty obvious I guess from the symptoms) but turns out that I can live with the default anyway (probably better than mine)... Think it is to do with moving from 1.2.3 to 1.3 that's caused the problem.
Added the extra code to make a page_1 and populate it... Broke the whole thing. Couldn't even see the home page... No idea why. Simply added the define it files.php and that is it ... boom. Exactly the same format for page_2 etc.. but no show.. so gave up on that for the moment.
Went to upload the site again and found ...
a) the useless hosting company had given me more space in one part of there system and hadn't rolled it over to the file manager so ran out of room with the images... took ages to convince them that it was actually their fault.. finally got onto someone who had half a clue and they fixed that.. (hence some of the earlier image problems).
b) discovered that I had accidently uploaded a version with page_1 enabled.. and bugger me if it didn't work on the hosting site (still doesn't on my test box).
Basically, back up again, customer mostly happy (still got some changes to make, but should be OK.. might have to ask some more..).
Thanks to everone who helped out here.. hope when my freshly ordered book turns up, some of these questions might already be answered.
Thanks again.
Peter.