when I turn the mixer tap to full hot water, it goes cold?

Hi, I have a Santon premier plus unvented hot water system, it used to run fine. But now when I turn the mixer tap to hot fully, the pressure drops and the water goes cold. I have to turn the tap off wait, then turn it on for it get hot, but i then cant turn it to full.

Can you please help and say why this is happening?