What do I need to check most when selecting a replacement for my existing mixer shower?

Check that the mixer shower you select is compatible with your water system i.e. low or high pressure. Check that the mixer you select is thermostatic in operation if you want optimum temperature control. Check that the inlet fittings on the new mixer offer the same measurement between them as the existing mixer, if not it will usually be necessary to alter pipework to suit. Check that the existing hot and cold supply will connect to the correct inlets on the new mixer i.e. hot on the left and cold on the right, if not it will be necessary to alter the pipework to suit.