You'll probably need to replace the shutoff valve.
Its strange you missed this result when you did your search to help yourself before posting here:
https://www.mirashowers.co.uk/support/fault-diagnosis/
Spares here:
https://www.showerdoc.com/shower-spares/mira (That site is well worth bookmarking. Also note: changing a flow valve is relatively straightforward but can be a bit of a pig sometimes)
xxx, February 2017
