I got pulled in Inverness over 2 years ago in my old Wagon, the cops told me I was allowed up to 92db.
It read 99db with their tester, i got a £40 fine. I paid the fine then after a bit of research I found out nearly all the local boy racers had also been caught and had contested the cops findings resulting in the courts not proceeding with any legal action. At the time there was no law about how loud your exhaust can be, I dont know if this has now changed.
The ticket I got said I had 'a defective exhaust'!!
The police also have to test the car in proper test conditions with no background noise etc, mine was tested beside a main road!