Sorry to hear about this fault.It sounds as though the 'thermal trip' is starting to activate.It's an internal sensor fault which can only be cured by replacing the 'engine' which is every single working part of the shower.
If you replaced the engine it would be a like for like fit with no...
It sounds as though the solenoid/plunger is the defective item.
As the plunger is pushed down it will engage the electric supply which
is then sent to the element which heats the water up.
unfortuantely the Gainsborough 850 is a discontinued model.It came in two variants 7.2kw & 8.5kw.The...