The overheat light is now on permanently, despite normal water flow. Water is cold. I note the advice of Roy in 2017 below and wondering whether you think it might work for my scenario...
The sequence of events today was as follows: shower was on and working fine, someone turned on outside water tap leading to zero flow and 'low flow' light in shower. We didn't get turning shower off for a few minutes unfortunately.
When outside tap turned off, and shower turned on, flow is fine but now the 'overheat' light is on and water cold.
I wonder if perhaps something has been irreversibly damaged and replacement shower would be necessary?