We had this sort of problem 40 years ago on the vintage tractor show circuit, at that time we gave it up.
Then last week this appears... http://www.telegraph.co.uk/finance/personalfinance/insurance/motorinsurance/11493891/Youll-need-100-car-insurance-for-lawnmowers-says-EU.html
The important bit being "The court in Brussels decided that any moving vehicle, on public and private land, should have motor insurance, regardless of whether they are driven on the road or not."