Man_Utd_Fan ... "I've only flew into MAN once".

Every cliche contains an element of truth

Should be no bother. I don't about shortcuts without exiting normally, but with only hand luggage its as simple as walking out of arrivals, going 5 strides, then up one flight of stairs and you're in departures. Should take less than quarter of an hour, although of course if LHR->MAN is delayed you might be cutting it fine.