Over Ealing? That's awful close to LHR for low level f/w traffic to be transiting. I say low level because it was raining hard according to theOP and rain severely attenuates aircraft noise so I surmise what he heard probably wasn't at even medium level. This is a very unusual place to find a fixed wing off the LHR approach.
Cub, at 0115, over London. It's a joke, right? Herc? On a tactical training sortie, just perhaps.
My guess is it was rotary. The S76 can sound very big-pistonish at times. Apart from finals to LHR or the remote possibility of a Herc doing something odd I'm at a loss to imagine what else it could be, given the locality.
ilesmark, how low would you judge it?