@macalter - Thanks for responding
I ended up removing all files except album and cache and reinstalled the same build but still had the same problem, so I took the plunge and emptied the admin tables in the DB and that did the trick, of course I had to enter all my gallery settings again but it didn't really take long.
Annoying thing is that I use the the MAMP environment on the Mac locally for doing all my zenphoto testing, changing themes and css before I put it live on-line, so I updated the build on 3 different installs with no problems before I applied it online. I guess you are right about a file not updating correctly, I think because the updating is so easy I have got a little blasé:-)
Yes my themes are customised too, as are my css. I use BBEdit to find theme differences, I also keep a list of all my changes.