Poor hot water temp at taps, temp fine at the boiler

I've had quite a few of the modern 'bar' type shower mixer valves with passing non return valves. The water leaving the boiler will be fine but the water at other hot water taps will be cold or luke warm. The problem is the non return valves allowing the water to mix within the shower valve. This can also be a problem with other thermostatic shower mixers.

The problem can be resolved by the customers plumber installing non return valves on the pipework to the shower.