You cannot say what the maximum payload is.
You CAN find out what are the maximum permitted weights
1. on axle # 1 the front axle
2. on axle # 2 the back axle
3. the whole vehicle maximum gross weight (no trailer) on both axles together, and please note, 1 + 2 might NOT equal 3 in this situation.
You can legally load it up until weight 3 is reached, as long as NEITHER weights 1 or 2 are exceeded in the process.
The lighter the vehicle can be made, the more you can carry, so if you have 100 kilos of tools and junk in there, unload it!
There must by law be a plate fastened to the car showing these weights, and as it happens, the maximum weight when towing of the whole lot.
If yours is a 90, you may be disappointed at the permitted load weight IN the car, but you might be impressed at the gross train weight with trailer, which may be well over SIX TONNES depending on your model of Def 90.
CharlesY