new design of recoil and petrol tank kit for championj E484 SP rotary mower?

The old combined fuel tank and recoil starter have been separated into two separate units by the manufacturers due to leakage problems with theold fuel tank. I have purchased the new kit and the instructions require the fitting of a new recoil unit on top of the engine which is secured by a nut. when attempting to undo the nut the whole assembly rotates. How do I hold the assemly stationary whilst I undo the nut. I wonder if I can fix the blade under the mower by some means which should solve the problem.