Thank you very much for your reply
The pump does not seem to work anymore, and for over a year. I intend to extend the house in two years time and decided to deal with the pump then.
Consequently the shower pressure is just normal, as if there was no pump. Hot water is running normally in the bathroom and kitchen.
Three years ago I removed an airlock by using a piece of hose pipe to connect the two taps in the hand basin, and running cold water into the hot water tap. I did this while the central heating was on. I do not know If this could remove what appears to be an airlock in the pipe that feeds hot water into the radiators in the ground floor. As I said earlier, the radiators in the top floor are fine.