If you have a cat3 equiped aircraft with full autoland etc,return to the departure field and land it.Watch the automatics like a hawk,but let the autoland do its thing.Set the parking brake and await medical assistance.This is a commonly debated scenario with the hapless victim normally an incapacitated captain.Two points;the situation you describe is critical in terms of life or death.The CAA would be extremely sympathetic provided the situation was dealt with sensibly.