Why would a shower cut off after just a few minutes?

Sounds as if the solenoid is closing off in the shower, suggest you get whoever installed unit back to check connections in the unit itself or possibly the switch outlet connections. If its an actual fault on the unit then contact Mira who have their own service engineers and they should then attend to rectify the fault. They will only come if its their unit that's faulty not the workmanship on installation.