Taken from Pistonheads No noise meter is needed. If in the opinion of the prosecuting officer, your car is louder than a standard car of similar type and age, an offence has been committed. Technically, yes if you fitted a pattern exhaust from, say, Euro Car Parts, it may well be non-compliant. Any modification to the standard exhaust (including changing for a non genuine one) may render you liable to prosecution. Personally, speaking as someone with a stainless system on my car, I think the risk of prosecution depends more on your driving style than anything else.