your PC may have acquired a 'spambot'
That's what I thought, especially as i just dump that sort of message in the spam folder and would never forward it. However, if I can find where it originated the owner will start to get loads of offers from a throw-away address.
I've cleaned out all my folders and re-installed Mail however I still get the error. Looks like it's going to the local Geek for some fettling.
Thanks for all the help