I'd would guess it to have been at around 20000 or so. Surprised if it was indeed the C-130J which are regular passers by on their way home to Brize, they seem to have a higher note; and as joy ride says this did sound very unusual - when I first heard it I did wonder if it was the AN-22 but as my fillings didn't start to rattle...
Ah well!
Thanks all,
Treadders