From CAP371:
A flight crew member is required to inform anyone who employs his services as a flight crew member of all flight times and flying duty periods undertaken, whether professionally or privately, except for flying in aircraft not exceeding 1,600 kg maximum weight and not flying for the purpose of public transport or aerial work.
Aerial work includes flying instruction for which the pilot is remunerated. It is also aerial work where valuable consideration is given specifically for flying instruction, even if the pilot receives no reward.
This sucks as it effectively means that an airline pilot cannot fly most piston twins (including the featherlite DA-42 which has MTOW of around 1750 Kg) on the odd day off, even for pleasure. It's insane. Surely the weight limitation should've been in line with aircraft weight categories, where the hell did this 1600 Kg figure come from?