Tow a car with a motorhome

I have an Elddis Autoquest 130 purchased new in February 2012 and have a tow bar fitted.  I am thinking of towing a small car using an A frame.  Does anyone know the legal requirements for this with regards to weight etc and can you advise how I ascertain the maximum towing weight for my vehicle.  Any other advice pertaining to the this topic would be appreciated.