Foggy. Closest I can get is that 15R/33L was closed in the late 60s (67ish?) and 23R/05L "two years later". This information is taken from a draft of a book "A History of Heathrow ATC", which may appear in print eventually.. Rwy 05(R) was closed in fairly recent times at the time when the 23 ILS was withdrawn and parking stands installed across the 05 threshold.
I'm busy for a fortnight but if nobody else comes up with the goods email me at
[email protected] and I'll try and find out some more.