They seem to be quite complicated when it comes to diagnosing faults.
We have a new Mira ATL shower. The old one went crazy, making all kinds of odd noises , showing 'low flow' then shutting down.
The Shower Doctor said they would look at it for about £70 plus parts plus vat which would cover the first hour of labour. The only guarantee they could give was that we'd have to pay the call out fee for possibly no result.
A heating engineer friend got us a new one wholesale and fitted it free.
Sorry I can't be of more help.