Skytrain Schedules
If my memory serves me right the schedules were published in the ABC World Airways Guide at the time. If you can pin down years/months you could check them there.
The company that published the guides is now called OAG, Official Airline Guide, and they are in Dunstable near Luton. They have an archive of old editions of the monthly guide.
If there still is such a thing, the CAA Library in Holborn also used to have old copies, you could try there.