Most interesting. I did not realise that 23L functioned so long. It could certainly have been used by the Manx 146 which took over from the Viscounts and started operations in December 1987. I remember it well as, by chance I was on the innaugural rotation of G-MIMA (I think it was) from LHR.
The aircraft was officially launching on the Friday evening rotation and there was to be the usual razzamatazz, I learnt. I had booked on the lunch time rotation to see my mother for the weekend and found myself with a small group of others being given champagne as the lunch time was a proving flight for the evening and the photographers. We could have popped in and out on 05R/23L with ease, especially as we usually went from gate 12 (or 14?) at the end of the long pier - this done for inspection and isolation of pax as IOM was a known staging point from Northern Ireland.
It turned out to be the only time I've been on a new a/c for a route but did find myself on the innaugral rotation of a new route once - but that did not involve EGLL.