Vaillent ecoMAX dodgy water temp?

I have a Valient ecoMAX condensing boiler, when I run the bath or shower the water runs hot for abour 3 mins and then get cooler untill totally freezing, if I turn the tap off and wait a min or 2 it then runs hot again for a few more min's and I have to repeate untill bath is full.....arghhhhh

If I only turn the tap on slightly the water wil run hot constantly.

The other day the heating didn't kick in, so tried to manualy overide it and it wouldnt work, hubby check and there was no pressure or water in the system so he refiled it and the problem with the water continues.

Would be greatfull for any ideas