Thanks TR.

Actually the problem turned out to be a Drupal upgrade issue, where (in my case) a complete copy of my Drupal folder and sub-folders turned up in sites/all/modules. So, to fix it I had to delete the duplicated folders and edit the system table to correct the module paths. See http://drupal.org/node/112884. After following the instructions update.php ran fine.

I checked the PHP memory size and in my case it is already set at 16Mb, but this is a top tip and something I will try to remember to check on the live site.

The site is still a bit sluggish though so I now need to try setting the query cache. Anyone know where I do this?

Edit: I've set up query caching but the site is still slow. I think I need to look at giving MySQL, Apache and/or PHP a few more resources...