Mira Orbis electric shower running after switch off

[markrobinson]

The model is J01B. The manual doesn't explain this fault. It is normal for the shower to run after switching off but usually for about 5 seconds. This is, I think, to purge the tank inside to minimise scaling up.
However recently it has started to take longer to cease flowing. Now it takes about 30 secs before it stops. Anyone know where to look to fix it?

 

[blup]

My understanding of the delay was to allow the temperature to reduce so the next user is not burnt by overheated water left in the guts of the shower. But new showers are sold without this feature. The delay is possibly down to a failing solenoid switch.