I recently bought my first home. We have now discovered that our terraced house backs onto a gym. This means that from the hours of 7am-10pm (Monday-Friday) and 10am-5pm (Saturday-Sunday) there is constant crashing and banging as weights drop off the floor which reverberates through every room of our house. I contacted the council and an environmental health officer came around and recognised it was a problem. However he told us everyone in the property before us had complained but that because the gym had been there for 30 years (compared to the newish flats) and that they had planning permission not much could be done.
Based on the info you have provided my advice is to escalate the issue through the Council using their complaints procedure if need be. Planning permission is no defence to nuisance, nor is coming to the nuisance. Businesses have a defence of “best praticable means” (meaning that they should take measures to avoid/prevent the nusiance) – this is the area that should be explored. In any case, it does not stop the local authority serving the notice if a nusiance exists (they have a duty to do so).