I have a shower over the bath tub upstairs in a 3 bed semi 'which I have been in for 11 months'it has 2 valves, one to turn the water on the other valve to get the right temperature, the hot water is provided by boiler which fires up when the hot is required, it's not an old boiler a valiant about 5 years old, the problem is the water pressure from the shower head is pretty awful I have tried different heads with no benifits can anyone help?