It seems the problem only affects some people's diaries..
I have added some debug code because at this point I've no idea what's causing it - everything looks exactly as it should be on the server.
Though I did have to chuckle at the hacker who tried to get in through the forum system - stopped by various other security measures in place to make up for the fact that phpBB is so shockingly insecure and is almost due to be banned from my servers forever!
Steve.