HI, MY HUSBAND HAS JUST TURNED OUR BATH ROUND AND FITTED MIXER TAPS. THE FIRST MIXERTAPS HE FIT THERE WAS NO PRESSURE AS WE GOT 14 TURN ONES. WE NOW HAVE GOT ORDINARLY MIXER TAPS AND THE PRESSURE TO THE BATH IS VERY GOOD, BUT WHEN WE PULL THE MIXER THE PRESSURES IS VERY POOR. ANY IDEAS WHY?
You have a flow rate problem not a pressure problem.
It sounds like the diverter in the bath/shower mixer tap. If the flow through the taps is good then that suggests the diverter isn't seating up (or down, or whatever design it is) properly. You can take it apart and see depending on how confident you are. Alternbatively perhaps you should get someone in to look at it.
However, if it's a recent purchase then talk to the supplier, say it's a problem and they may well replace the divertor if not the whole tap assembly. They might even send someone along to do the swap if they are feeling generous.
Incidentally you have just discovered that 1/4 turn ceramic taps really restrict the flow when the pressure is around 0.25 bar (ie what you'd expect from a 2 metres head between cold tank in roof and the bath tap.