Economy 7 heating peak switch trips my electricity

Hi. I have economy 7 heating. When I switch the peak switch for emergency hot water (once the stored hot water runs out) this trips my electric fuse box and shuts down everything. I can only switch the MCB switch once the peak switch has been turned off. Lately my store hot water is tsrating to run out quick. I.e when I have a quick 5 min shower the water starts getting cold. I don't consume a lot of ot water. A few people have suggested the thermostat for the boiler or the immersion heater element. Any advice would be greatly welcome as I'm dumbfounded