you may have temperature up too high so the thermal switch is cutting out. Also check your shower head to make sure it's not limescaled up. If all else fails ring Mira's helpline on 0844 5715000 and book one of their fixed price repairs
check the tco for continuity.
hi-spec plumbing & heating (essex)
I can answer this question...
I have a mira 10.8 kw shower and it has stopped working the water runs fas?