Sorry Chuck it was late when I first posted.
More info then:-
I copied all the 5.5 update files to the correct folders on my 5.3 installation. I then ran the upgrade.php file and received the message in my above post(this was on the first run of upgrade.php). I then tried to access my site and received a white but blank screen. On reading some posts on this forum I though maybe I should copy my original config files back, but this did not help, I ran the upgrade.php again with my original config files, no different.
If I add the index.php to the end of the url I receive the following message,
Fatal error: Call to undefined function: update_whoonline() in /homepages/5/d161531532/htdocs/photopost/pp-inc.php on line 484
My installation is at http://www.trackaction-online.co.uk/photopost/
PS: My forum is phpbb and uses the same mysql database, I didn't close the forum before upgrade but I did close photopost.