About 3 weeks ago i did my cross channel check with my instructor and his nephew. His nephew flew out, and I flew back
On the basis you flew back (and you're using the same A/C), presume you still have the W&B calcs and PLOG you used for that flight?
Therefore you know the empty weight, you know MAUW, you know how much fuel you need (and its SG) and you know that 4*14ST = 784lbs...
Does that bring you in under MAUW?
The only other part of the equation is where the CG will be, but I don't think you'll get that far in your calculations
(NB, not because you can't do the maths, but 4*14ST persons plus fuel is a lot of weight)