The on off switch will only operate if pressed really hard and has to be continually pressed to keep the water flowing and even then stops and starts.
Sounds like you have a faulty Latching Switch. Removing the cover on an Electric Shower should only be undertaken by an Engineer or Electrician. Why don't you ring Mira Customer Service 0844 571 5000 and they'll be able to arrange a repair for you at a reasonable fixed price
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How can I fix my mira shower?