How do I proceed against a garage that overfilled my car with oil during a filter change?

I recently had an oil filter change on my 2007 Mercedes-Benz 300SL. The EGR light has come on. I've checked the oil level, which is very high (about 1cm over maximum), and suspect that the catalytic conveter has been damaged. The garage is refusing to acknowledge this. How should I proceed?
In a petrol engined car the only reason for the engine oil level to be excessively high is if it has been overfilled with oil, so you have a strong case against the garage that performed the oil change. Tell them if they don't capitulate you then you'll take the matter to Small Claims.
Answered by Honest John on

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