To confuse matters the law changed fairly recently for post 97 drivers, you may now tow any combination you like on a b licence as long as the actual physical weight (ie the read out in kg from a weighbridge) of the combination doesn't exceed 3500kg, or you may tow up to 750kg mam behind up to 3500kg mam as per the old rules...
It seems to be an attempt to simplify matters, I emailed the dvla to check and they confirmed the above, I took my 110 csw to the weighbridge, FYI it was 2400kg with me, half a tank of diesel and all my ropes, hi lift and toolbox on board, my unladen sankey was 360kg, so under the new rules I can carry up to 740kg of treasures and be legal... Full details on the direct.gov website!
Mike