Can anyone advise a good web site for downloading free good quality midi files?

It can be a mixed bag out there but there is some nice music around as well. All the links I know of are listed on http://www.synthzone.com/midifile.htm You'll just have to explore for a while.

The MIDI file that plays when you load that page is from http://www.keybdwizrd.com/ Michael's MIDI compositions have a very nice groove and melodic layering.