Given this probably affects a huge number of us on this site and many other sites, I've had a petition to No 10 approved asking for a review of the recent legisliation as per the petition details I have posted below.
Feel free (and please do!) pass this link around any other car-related forums you use, perhaps others too.
"Recent legislation has increased the powers of The Police Force to stop and fine drivers of cars where they believe the exhaust system fitted is louder than the original factory fitted system.
Whilst we appreciate there is a minority of car owners who intentionally fit an aftermarket exhaust for the increased noise levels alone, the majority of car modifiers choose an exhaust that offers a performance increase whilst keeping the noise levels as acceptable as possible. As few car manufacturers provide the factory fitted exhaust noise in decibels, it is impossible to know if an aftermarket exhaust system is louder than the original or not.
Consideration must be given to the relative noise of an aftermarket exhaust system vs. the large number of cars which, from factory, have very loud exhausts. Aftermarket exhaust systems are still likely to be significantly quieter than drivers of such cars, yet only the drivers with aftermarket exhausts can be fined which is unfair.
The only fair solution is to introduce a testable upper decibel limit for any fitted aftermarket car exhaust system. A testable limit would make a much fairer alternative to the current objective assessment."