How do you determine the maximum authorised combination mass of a vehicle combination consisting of a towing vehicle with rigid drawbar trailer?

You answered correctly. When calculating the maximum authorised combination mass for a vehicle with a rigid drawbar trailer, you subtract the vertical load of the trailer from the maximum authorised mass of the trailer, but only if this load is higher than what the towing vehicle is allowed to handle. This ensures that you do not exceed the legal limits for the vertical (downward) force on the tow bar, which is important for road safety and compliance. In summary, always check both the trailer and towing vehicle limits, and make sure the vertical load does not exceed what the towing vehicle can safely tow.