Thanks Ross.
I'm basically getting the 1142 INSERT error on any table I try to optimize, even when doing them one by one and small ones first.
I'll try dropping the wp_commentsmeta table I guess as that's the one causing the problem and it's not even got anything in it now anyway! I've got a back up of that table specifically that I just did, so I'll see how it goes!
I'll give this a go a little later and report back...fingers crossed!