Help - bizarre hot water problem!!!

I "lost" my hot water today, it's not a boiler or immersion heater failure, but a very weird hot water distribution problem.
I open a hot water tap on, and the water runs ice cold, just like the water out of the cold water tap. However the pipe from the hot water draw off on the hot water tank goes warm as soon as the tap is opened (and cold when the tap is closed) so the hot water goes *somewhere*, just not out of the tap!
I then open a second hot water tap, and after a few seconds I get hot water out of *both* taps. I then close one tap (doesn't matter which one), and after a few seconds the water runs cold again!
This is a new installation with an unvented hot water tank and a Glowworm boiler. Any idea what might cause this???
Thanks in advance!
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Nothing to do with the boiler. Did the new installation involve new pipework? Did the installers leave old pipework joined to the system? Sounds like a dead-leg problem maybe? Get the installers back.
Angus
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