I don't think it is hard disk - but please don't think I speak with either authority or conviction. Philippe de Rycke pointed me towards some HD inspection progs but they haven't turned up anything alarming yet - as far as I can see.
Thanks for your tip about syslog. I had a look at it and what I did find out was that something called gconf has star billing in syslog. I don't know what it does but it doesn't seem happy! Tomorrow I'll try to find out more about it - but in the meantime if anyone can tell me what and why it is I'd be dead chuffed.