As some of you had worked out I'd not gone completely crazy and thought I'd add even more weight to the car than was already taken up with boat gear, suitcases, people etc. Both tanks were filled from a previous launch where the 3 of us on board had a way to go that was aborted 10mins in.
Good news is I took the opportunity to launch yesterday in Poole as the weather was too good to not take advantage of, so I'm now down to 1 full tank and about 5 litres in the second. Did a round trip to Lulworth that burned off most of the first tank.
Good to know it was never a requirement though. Thanks all.