you can tow up to the total weight in the plate on the cab, this includes the weight of the trailer plus the load it can take, so include the merc pick up in that figure.
"If you passed a car test on or after 1 January 1997 you're limited to vehicles up to 3.5 tonnes maximum authorised mass towing a trailer up to 750kgs, or a vehicle and trailer combination up to 3.5 tonnes MAM providing the MAM of the trailer doesn't exceed the unladen mass of the towing vehicle. You will need to pass an additional driving test in B+E if you wish to tow a trailer combination which exceeds these weight limits."
