En-suite Shower problem

Firstly, if you have hard water it is very likely you need to de-scale the mixer.

Is it fed by 15mm pipes over a long and tortuous route?

Do you have a header tank for your HW in the loft? If yes, and this is mounted on the loft floor you can help a bit by mounting it as high as possible. Alternatively you could add a booster pump at an all in cost of ~£150

One possibility - you may have the wrong shower fitting. Some are designed to work with high pressure system (such as mains pressure or pumped) and others for lower pressure tank (gravity) fed systems.

I presume the flow from the bath taps is fine in the en-suite? Also that the feed to the bath taps and shower is common till you get near the bath? If so, the flow in the pipes is not the problem. It is the mixer.

Has it ever worke well? If performance is steadily degrading then it may be gunged up. If it has never worked then it may be a high pressure mixer valve which requires a minimum pressure to work correctly.

Any manufacturere and model information on the mixer? If so, a Google may help.

If it is the wrong mixer, than as suggested elsewhere one way to solve this is to fit a booster pump to give you a power shower.

HTH Dave R

Quite frankly, a gravity fed mixer valve is never going to give a good shower. Even if the flow rate is there, there isn't the pressure to blast the water out. You're going to need a pump (or a mains pressure hot water system).

Christian.