Aqualisa very hot water low pressure

[astridoasis]

Good morning, The first 2 times, the water was very hot but running against the wall behind the shower and it was to do with the pressure relief device. For previous occurrences, I just replaced the part but this time I have replaced the part so it's not leaking anymore but I am having the same kettle effect and the water is either really hot or cold with no pressure. There is no issue in the house in terms of pressure at the taps. I would like to replace this shower as I am a bit fed up with it (it seems that every time I pull the hose at an angle it creates the issue) so I was wondering if someone could tell me what shower would fit the same plumbing as this model (Aqualisa Quarts 8.5), please?