hi it sounds like you have an airlock on the hot water side and its possible its being caused by the pump fitting on the shower if its not been fitted with a surrey flange off the cylinder top, as otherwise it can pull air in through the expansion pipe in the roof, so get the instalation checked out first.
The airlock can be got rid of by connecting the cold mains water to the hot tap usually on the sink via a plastic hose with a tap connector on each end then you can open the hot tap then the cold and backfill the pipework until it comes out of the expansion pipe then it should get rid of all air
I have a mixer tap on the bathroom sink and I was able to force cold water into the hot side by blocking the tap. I will have to look at the shower pump and see if I can do anything to avoid the problem in future, but for now I am sorted.
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