Was it ever actually called "15C" ? In other words, was there a time when Heathrow had three operational 15/33 runways ?
I'd always been under the impression that the original 15R/33L (Runway No 3 in the "RAF" schema) was replaced by the new 15R/33L (Runway No 6) to make room for the Central Terminal Area at the same time (late 1940s) as Runways 4, 5 and 7 were added.