Christian McArdle wrote:
a) my electric shower now runs fine,with no need to turn it on for approx
5 minutes,

As for the electric shower, if I had a combi boiler, then the electric
shower would already be in the bin. Replace it with a thermostatic pressure
balanced mixer shower fed by the mains cold and hot water.

"no need to turn it on for approx 5 minutes" means to me that the combi
isn't quite adequate in terms of temperature.