A bit late to post on this thread perhaps, but someone might be interested:
I was looking for 3rd party insurance to simply make me road legal - nothing more as just too many other out-goings presently & tractor is well secured when not in use. The point is though that my broker offered me a KGM policy at some £83 which was about a third of what others were offering. I doubt that that differential will read across to "full" tractor insurance, but I wonder whether KGM policies might be worth checking out.