I can't think what it might have been. Do Air Algerie still fly C-130s into Heathrow? But then it wouldn't be coming from the north. Are the freight Electras still around? Otherwise we're down to something like an AN-12..
You could check here:
lhr nov 2010 in a day or so when the movements for 5th up up. I don't understand some of the info, but one column certainly has the type and I believe the last but one column is the time.
Somebody is bound to know...