Building Services Engineer

5 years and 7 plumbers and one chartered Surveyor later no one can work out why my shower doesn't work properly and Surveyor recommended getting in a building services engineer. I have lovely bathroom with a shower that dribbles and has no pressure. Water board says pressure into the house is fine and indeed its not a problem elsewhere.......I know its not a huge job but it would make a big difference to me if I could get it resolved! Im in Birmingham UK