It's done through NSALG - a company called NewSpace. Last year I spent several days phoning around to get a better deal and couldn't find one. You have to be affiliated to get the insurance, and that in itself costs £2 per head - that's why we charge £4 per member supplement (regardless of how much land is rented).
But then members can get their £4 back by putting in an order for King's Seeds. (And - although it was the first time we'd ever done it, and not many members ordered, we still made nearly £60 profit for the Society!)
Details are in the NSALG magazine (or pm me).