Apologies, I was more stupid than usual yesterday
I failed to copy the disposition line for the last SendFile correctly. It actually reads

 disposition => "inline; filename=\"picture.gif\";\r\nContent-ID:<ed1>
+",
[download]

I took out the error checking to keep the problem as small as possible. I'd already checked that there were no errors when the script ran. Sorry, I should of said. Still can't get it working. :-(