The categories are shown on your drivers license. If you only have category B on your driving license then you can only tow a unbraked trailer up to 750kgs. To tow a braked trailer (plant trailers, stock boxes e.t.c) or caravan you must have category B+E on your license. All drivers who have passed there car test before the 1st of January 1997 will have category B+E on there license. Any driver passing there car test after 1997 will have to do an additional test to gain category B+E. I did the B+E test last year, if your used to towing trailers it'll be no problem. Test takes about 1.5 hours and involves a reversing manouvre and a uncoupling/coupling back up exercise.