I've just got hole of a copy of the Pooleys 1965 flight guide and it shows LHR having 5 runways and says the sixth was in use as a taxiway.
28L/10R
28R/10L
05L/23R
05R/23L
33/15
(33R/15L - in use as a taxiway)
I was also surprised to see that Prestwick had what looks like six runways as well, again in the 1965 Pooleys guide.
Gaz