Righto you fat-fingered people.... it might have been my fault after all.
Question 1) Did this only happen with significantly large messages?
Question 2) Was a photo copied into the text box?
Question 3) Is it now working ok?
I increased the maximum data limit from 0.2mb to 50mb
The way I created the problem was by copying the entire BBC news HTML into a new message to myself. It failed, then i tried again with an increased limit and it worked ok