What Car? guide to... towing - Towing licence
If you passed after this date, you're restricted to a trailer with a MAM of just 750kg, although you may tow a trailer with a higher MAM if this doesn't exceed your car's unladen weight (not the kerbweight) and the combination of MAMs doesn't exceed 3.5 tonnes.
If you need to tow more than this, you'll have to pass another test to get the necessary Category B+E on your licence. Contact the Driving and Standards Agency (DSA) - www.dsa.gov.uk.