Tim+ has brought this to us :
Long term answer may be to just go back to external H & C taps but he does
like the pre-mixed warm water.

Any suggestions?

What is the height of the header tank, above the tap?

I fitted one on a downstairs toilet wash basin 8 years ago and its been
absolutely fine. Header tank is in the loft of my semi. I recently
adapted it to provide an outside shower facility for the dogs on my
drive. All works great..