Hot water cools when tap is opened fully?

Hi.
I have a Vaillant Turbo max plus 824E, that works fine untill the bath hot water tap is opened fully and then the temp drops to about 28deg. If the tap is only opened half way all is fine. There are no fault codes displayed and the lights on the front are correct.
This is not what I need after Christmas, but does anyone know what I might be looking at?
Thanks.