Just one pilot allowed?
 

Awhile ago I was on a flight with bmi and we were approx 10 min from heathrow when the co-pilot left the flightdeck to sit out the landing in business class, now I am almost certain there was not a third crew member in the cockpit that would of explained it maybe.If not, is there any particular reason for this action or is it even allowed?
Also can someone clear this up, do the pilots usually handle either communications or aircraft manouevering, will the f/o for example be tasked with both or just one of these tasks. Therefore how are the tasks usually delgated and is this then set in stone for the duration of the flight.

You are mistaken, there would have been two pilots for landing (and take-off come to that).
The division of duties between pilots are roughly thus:
"Pilot Flying" flies the aircraft-either hand-flying or through the autopilot.
"Pilot non-flying" handles communications.
There are some "variations on a theme", but the above is basically "industry standard".
Hope this clears up any confusion you may have over what actually happens during a flight.

And to save any confussion, yes each pilot generally takes it in turns, i.e Capt flies as PF for the first sector and F/O flies as PNF, on the return leg they swap.

F/O Acts as Captain when it is his leg making all the decsions BUT the butt always stops with the Captain. i.e if the F/O screws it up it is the Captain's fault.

bmi do not have a very good crewing department so crews spend alot of time passengering around the network. I have absolutely no doubt that the F/O you saw was not operating but had just gone in to see the operating crew!!!!!

Happy flying