I have had fitted a thermostatic shower mixer valve but I cannot adjust the temp it is either scalding hot or cold. My plumber said the valve was faulty so I replaced the cartridge but it is still the same. Could it be that if he has connected the hot and cold feeds wrong it would work like this. If he has then I will have to knock an hole in the wall to get at the valve from the back as it is tiled into the shower!

Without knowing make of valve, the problem could be caused by the balancer. Some valves have balancer built in cartridge and some have separate balancers such as Kohler valves. Try taking out cartridge and shaking it and see if you hear it rattle, if it does it should be OK. Normally if the balancer is bad you will not get any water, but not always. If he connected the hot and cold wrong the water would just be crossed, but would still work.

Give us name and model number... also take a nice sharp photo and post it for us. Back to you. Milo

Unfortunately it sounds as though it has been plumbed in with the feeds the wrong way around. If this is the case it would always have been so. Was it working before? I suggest getting the plumber back to fix it as he should not have tiled it all in place if it was not functioning as it should!