MAN is using 05L for departures at the moment. A Virgin 747 flying transatlantic would climb straight ahead until it is just about over Stockport then turn left (i.e. to the North-NW of the airport). The block on these departures is 5000ft (I haven't done a westerly off 05L for a while though) and a heavy 747 might take all the time available to reach that block. Alternatively, it could be headed north to GLA (POL departure) if it is the MAN-GLA-MCO flight. So, I expect what you describe will simply be the aircraft on the climb-out/departure. Aircraft do tend to have periods of low flight when either approaching or leaving the ground you see.