There used to be six runways at Heathrow - three further diagonals. That was in the days when it was more critical than it is now to take off and land exactly into the wind, so it was in fact three pairs of runways. The present two, albeit shorter in those days, the 04/22, another at the same angle at the west end of the airport and two further diagonals at almost right angles to those two and crossing them roughly in the middle.
There were originally three runways that ran northwest/southeast (I don't know the exact bearings, something like 33/15), but I don't know if they were ever used. The centre one was built on pretty quickly, when the central buildings were put up. To my knowledge, runway 23L/05R was in use until relatively recently, usually only for landing when there was a very strong southwest wind, but I can't recall 23R/05L ever being used.
Looking at the original plans, there were ideas for another three runways to the north of the Bath Road. This is why that area was not developed. Even though, with the concentration of traffic on the then 28/10 (now 27/09) runways, plans for two of them were dropped, the option for a third 28/10 (27/09) runway has always been there.
One of the ironies of the situation is that Sipson, Cranford and Harmondsworth have only survived as nice Middlesex villages because expansion of Heathrow north of the Bath Road has always been an option. Had the idea of expansion north of the Bath Road been fully abandoned, that area would have become part of the suburban sprawl down from Hayes and along from Hounslow.