Hi Joe,
I checked the entries in the mp_IndexingQueue table again & they all seemed to be OK! Despite this I manually deleted them (there were over 130) & turned "DisableSearchIndex" back to false (By the way I tried enabling the <add key="ShowRebuildSearchIndexButtonToAdmins" value="true", but I couldn't find the button to rebuild the search index - where does it appear?).
Anyway, after re-enabling the Search Index I checked the mp_IndexingQueue table again & it's just showing nulls... and all of the \sites\(sitenumber)\index\ folders are now having files written to them! I guess that there must have been a problem with one of the entries (even though I couldn't spot it).
As before, I'll let it run for a while & see if the errors have stopped. Thank you very much agin for your help, it's much appreciated.
Regards,
Craig