Right I've solved this, but I don't understand why and I'd like to understand if possible.
I made the cardinal sin of changing too many things when I moved servers.
I changed php to allow larger uploads and changed the following:
The idea being to allow much larger pictures to be uploaded as this server was much more powerful.
When looking at the pictures in my bulk upload folder there was a large one for 4Mb. Deleting this allowed everything else to work fine.
So what's wrong with my php changes above and is there something I've missed which is causing Photopost to baulk at the 4MB pictures.