Does anyone know what the law is regarding levels of noise coming from bars and restaurant ?

If You Rent A Place in the town, there will be noise..

Noise levels need to be reduced to an "acceptable" level after midnight. If a party is causing noise disturbance after midnight, you can call the police to intervene.

If you file a complaint with town council about noisy equipment like heat exchangers or extractors, they will come and measure noise levels around your property to determine wether you are right and subsequently order the owner of the equipment to either install noise dampening or relocate the equipment.

I stayed a couple of nights in Feb half term (the SOL) the noise from Sully's was VERY loud, 3.30 am

In Turkey the bars have had shutters fitted to keep the noise in. After midnight the Gendarma walk down the streets and all along you can see/hear the shutters closing! Any bar that is too loud can be shut down for several days if the Gendarma say so. It works very well.