Hi I have a Mira Zest 7.5/8.5 kW shower (going off the instruction manual - not sure which one it is sorry, it was already here when I moved in a few years ago.) It had been working fine up until a few months ago but now if I set it to High the water will go cold after about 5-10 mins of use. Low and Medium work fine. Also it does seem to cut out sooner if I increase the temp when on High. I've cleaned & descaled the shower head, opened the mains valve fully outside the house (It was on about 3/4 fully open) but this has had no effect. I've tried leaving the shower running after it's gone cold but doesn't get hot again unless I turn it off for a bit. I assumed it was a heater element problem but could it be dodgy wiring or something? The house is pretty old. Long and short of it is should I replace the shower or is the problem electrical? Cheers!