Sale of used car buyers rights

[dl56uk]

I bought a used car from a car showroom on the 28th of june 2009 with a 12 month MOT . Today the car broke down exactly 29 days afte purchasing it . The AA cannot find anything wrong with the engine, and have checked everything, but it will not start. They have deduced that it could be the catalyst that as collaped inside the casing and casing a blockage in the exhast and not allowing any fumes to escape thus stoping the car from firing. If this is found to be the reason and i have to have a new exhaust system, have I any rights after 29 days to ask the garage i purchased it from to pay any bills. The catalyst was passed by them in a MOT test if it went wrong after 29 days have i any rights to ask them to pay 4 any new parts fitted ? On the invoice it do's NOT say sold as seen. Please advice to what i can do .. Thanks