Just remember not to confuse unladen weight, actual weight, and MAM.
The MAM of the trailer you can tow if you only have CatB is 750kgs.
2245 is the max gross weight/MAM of the vehicle without trailer attached. Thats the designers max weight for the car, its fuel, passengers and anything else within it, that the brakes, suspension etc are designed to cope with.
As soon as you attach a trailer, its then the train weight that is used, ie 3845. The combination then becomes the all up weight of the car and contents plus the trailer and contents.
Easiest way to establish how much you can put into the trailer is to use a public weighbridge. Take car and empty trailer, complete with passengers and full fuel load and weigh the whole thing. Subtract the figure from the train weight and thats the max the trailer and car can carry, as far as the train weight goes.
That process is using the Manufacturers restrictions, not DVLA regarding driver licencing....
Weigh the trailer on its own and subtract that weight from 750kgs. The resultant figure is the max load the trailer can carry and is probably less than the calculated train weight figure......
Remember that the construction of the trailer itself must not be overloaded, nor the applicable load rating for the tyres, nor the manner the load is carried such that it overloads the rear axle weight of the car - 1230 kgs....