My PC was playing silly buggers with me. Wouldn't play the MP4s I'd recorded on my phone or the 3GPs that Grant had. Spent hours faffing with this that and the next until finally I installed this:
http://www.free-codecs.com/download/K_Lite..._Codec_Pack.htm
It'll ask you to remove any other codecs you have installed becaue it will replace them for you, which I answered yes to, and it will also run a wee tidy up on your PC looking for out of date or broken ones (the cause of my problems).
Then bingo, MP4s and 3GP working no problem. As well as divx, xvid, 3ivx etc. etc. etc.