My car exhaust is blowing in front of the CATS where two joints meet and the clamps have corroded so a joint is not sealed they look very corroded. I took to a local exhaust centre and they said all 4 cats need replacing as the joint is not repairable. Have got my car booked into a main dealer for an estimate. What are my options ? Can a local exhaust centre repair? or does it have to be a main dealer? and what is the likely cost?