Ok I decided to go with a set of four 6-12 Trelleborg tyres so off my letter went to Santa, as luck would have it he's busy later in the month so he dropped them off early.
The trailer and a spare set of tractor rims were sandblasted and a bit of MF yellow splash on, then I had the joy of fitting the tyres to them. This took a lot more effort than I remember, the side walls and the beads seemed really hard even after warming them all day.
Tyre Warmer/Play pen
Comparison of 6-12 and 7-12, there is a much bigger difference than I thought there would be.
New Tractor tyres
And at last tyres fitted to the trailer
Now I just need to get it hooked up and take it for a spin.