It's pretty quiet here at night and often it's possible to hear overflying aircraft well above 30,000'.
At about 0415 I heard jet engine noise from great height so went to FlightRadar 24 on my iPad.
By the time I managed to take a screen shot the aircraft in question was some miles away over NW London...
I know that specialised cargo variants of large aircraft are developed from time to time but this one had escaped my attention.
Here's the beast in question.
Boeing Dreamlifter