Dave Bryant said:

MIDI to WAV/MP3 is a doddle - it's just like recording a performer/group/orchestra playing from a score.

I was asking if he knew of any software to do this. This is almost the reverse of the original question of the thread.

Anyway, I decided not to be lazy and did a search and found this - a plug-in for Winamp that does the job. With a little messing around with the plug-in configuration, I was able to make it work.