Aah, had a poke around and can't locate anything resembling a pop3 folder. Appreciate your help but it really all seems a bit too hard. I'm just gonna start again with the fresh install.
If anyone's still interested, I deleted every email account, rebooted and then re-added one. Same problem. Something in that install of Opera has cache'd messages that I've long since deleted from the mail servers.
Anyway, thanks again.