Newly fitted GFCH system with Glow Worm 24cxi boiler. CH is fine, the hot
water (with the boiler on the recommended ECO setting or even turned up to
maximum) will be at a sufficient temperature downstairs, but for the two hot
taps (sink and bath) upstairs the water only comes out lukewarm, even whilst
running the tap for a while, the temperature remains constant. Also, if both
these taps are turned on fully, the water will only come out of the bath
tap, which is the lowest fitted. What could be the problem, and how to
rectify?

Marcus