Well I finally tracked this one down.
There were no badly formed <IMG ..> tags but I do have to say thank you Laurie and Wendy for the hint anyway as it pointed me in the right direction. Turns out I'd managed to put in a bogus option on a <TD> tag. What I should have written was <TD BGCOLOR="#123456"> but what I'd written was <TD BACKGROUND="#123456">. As the browser expects an image URL in the background option it was treating the color triplet, specifically the # at the start, as a bookmark hyperlink instead and therefore tried to do a GET on the originating action.. Yes, I realize I should have done the table cell background color stuff in a stylesheet. Was one of those late night couldn't be bothered type errors. You would not believe what a complete bitch it was to track down though! Regards Marty