Good advice, thanks guys. I have about 40mm under one bilge runner when the other is in contact with the trailer, but I might be guilty of strapping her down tighter than Sweibertje suggests and I certainly tie the painter tight. The beam across the boat solves a couple of problems; ensuring the pull is downwards and not squeezing the gunwhales inwards, also spacing the ratchet attachment out and away from the hull. I will make one.
As a point of interest what sort of pressure in the tyres do you use Sweibertje? I have been advised to pump them up hard (50psi, about 4 Bar) and it occurs to me that at that sort of pressure there is not much give in them. I have 8 inch wheels and will be changing to 10 inch over the winter. I have found when they were at around 2 Bar the tyres were getting hot.