Why did the exhaust fall off?
Exhausts are held in place by clips and hangers. If the clips or hangers were corroded, then it could be argued that it was your fault for failing to maintain the car. Think it's a traffic offence to let things fall off your car.
If your argument is that the exhaust was improperly secured by the workmen, either because of faulty materials or poor workmanship, then you would have to prove that this was the case.
As johnny37 says, go back to the garage. Double-check whether you instructed them to carry out specific work (ie. I want you to fit a new exhaust), or asked for their experts to check and recommend the work required (ie. we've 'ad a look as you asked and your 'ole exhaust needs done, mate). In the second instance their 'expert fitters' should have noticed any problems with the clips/hangers and advised replacement.
Your argument would be that you bought goods (the exhaust) and services (the expert advice and fitting). One or both were not of satisfactory quality, leading to the incident.
Also check whether the guarantee applies only to the goods (ie. the exhaust) or to both the goods and services (ie. the fitting as well).
As for jno's scenario, the second car and the bus should both have been travelling at an appropriate speed and distance to allow them to take avoiding action. The Highways Agency should have protected traffic from falling over the cliff with an appropriate barrier. The explosives factory should have carried out a risk assessment to reduce the risk and impact of such an accident. The local authority shouldn't have built so many homes near an explosives factory.
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