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.
RAF Runway No. 3 was designated 16/34. It was decommissioned after 2 years of use (1945-47) to make way for the civil 'Central Area'. 'Waite's Alley' was never a 15C/33C, it was the northern stub of 16 which was painted over as 'taxiway'. The word remains visible on Google Maps. Loads of stuff on all this (and the 1946 ten-runway proposal...!) in Philip Sherwood's book 'Heathrow-2000 years of history'.
"The [16/34] runway....had to be built to keep up the pretence that the airport was needed by the RAF...Its construction was opposed by BOAC because it was entirely unsuitable for civil aviation and it was subsequently abandoned...That this runway might eventually be redundant to the civil scheme was known when construction commenced...the period of use of the redundant runway will be less than two years. Its cost will have been £350.000 [at 1946 prices]"