need advice

[nttrucker]

Just moted car and it failed on the emissions the mot tester said if I change the cat it should pass. Well bought a new cat and tried to fit it but when I've put it in the joint between the cat and the first silencer keeps coming apart and the clamp the shop supplied won't hold it together and am getting really fed up with it has anyone else had this problem and what did they do to sort it out.