Slight cock-up in my previous post - the pilot offering me the flight would be Commander for the whole flight but as neither of us would be operating under the supervision of the other, in the 2 hr flight we would only be entitled to log P1 time for the actual time each of us was controlling the ac - and it would add up to 2hr between us. But he would be the one your CAA man would speak to as Commander if during the period of his command the other pilot had the airprox you describe.