So - how can I play the video files back on my Sony Ericson k750i?

Try recoding them with something like Super (free) from
http://www.erightsoft.com

Its a pain in the ass to find the download link usually, with multiple
pages of spiel telling you what it can do (and it does _the lot_) but
hopefully this will help you get there quicker...

http://www.erightsoft.net/S6Kg1.html

Scroll right down to the bottom for the download links...