Thank you, martin102, I seem to have plugged most of the gaps of missing data now. The only number missing now is a Dan-Air flight on 06.08.86 LHR-INV DA ??? (dep 20h00). At a guess, it could be DA158, because 3 years later on 25.04 89 I did INV-LHR leaving at 17h30 arriving at 18h40 and it would be logical to assume that it turned around and became DA158 for the late evening flight to INV.
Similarly, in 1990 I flew on 06.04.90 LHR-INV DA 152 (dep 09h00 arr 10h40), which gave me the number for the same route, same timing, on 27.01.89. I am just assuming that the number did not change here.

Lastly, in my researches in the early 1990s, there are a couple of missing or dubious numbers. Here they are:

17.07.92 LHR-EDI. BD 056 (?) (dep 12h40)
17.07.92 LHR-INV. BA ???? (dep 17h00)
24.08.95 LHR-GVA. BA 730 (?) (dep 15h35)

For the first flight, I took a British Midland flight at almost exactly the same time some 15 years later, 12h45 instead of 12h40, with the same number. Bit of a stretch, but there is a good chance the number remained the same.
For the second flight from LHR to INV leaving at 17h00 the number is completely missing. It's just a couple of months after the Dan-Air takeover, I think, so maybe the number can be extrapolated from a Dan-Air one.
For the third flight, I'm not quite sure why I have a doubt, but I think BA730 was the number of a similarly timed flight I took another time. But I actually can't find another BA730 in my records. I do remember that the Swissair flight went out at 15h05 and that timing remained in place for years.