Ken,
I had a google of the flight today, and wikipedia claims that since mid-2007 the flights have been done by Omni Air DC-10's, but on the tristars have been used sporadically as well.
The 'Mount Pleasant RAF' page on wikipedia also says that the Antanov 225 aircraft are used at times for large cargo as well
http://en.wikipedia.org/wiki/RAF_Mou...senger_flights
Nick