ok man you've been a big big help! there's a programmer within you just waiting to burst out!!

I just checked the maya help file about the pre and post mel commands and this is what is says:

A MEL command or script to run before rendering each frame (Pre Render MEL), or after rendering each frame (Post Render MEL).

Which is just not true, it just runs it once and then then doesnt do it on any other frames.... damn it!!

ALan