Sounds like a problem I had. The supplier of an Elite mixer I purchased did not mention that it would not work with a gravity feed hot water system. Plenty of cold but only a dribble of hot. I had to install a new mains hot water system. This gave equal balance of pressure to both sides of the mixer unit. This is essential for mixers. Replacing all the 60s rusted gal pipes and corroded copper pipes throughout the house with Sharkbite fittings and PTE plastic tube gave much better pressure everywhere with clear and clean tasting water. We put mixers in two showers, two vanities, kitchen and laundry and all work perfectly. You can buy all the sharkbite fittings at Mitre ten or Reece. Its a brilliant system and may be installed by any practical and intelligent soul.
malcolm
August 2010