In case it's of interest to anyone, motorbike parking is free at T1/T3, also at T4. Directions how to reach the spot on BAA website. Helps if you are only carrying hand luggage suitable to bring and get home on a motorbike! Haven't parked at T5 yet but presume this will also be free. There is a staff parking area that is not documented, the ones on the BAA website are the official ones for those flying.