Further investigation
the banging only really occurs when the 3 way valve goes from feeding the CH to combine the HW and CH
the Braukmann ea122 Valve appears to work ok
no water is going up the vent pipe
Moved into a new house on the 11th Jan
since then when the heating/water are on
there is a loud banging when the thermostat on the cylinder calls for heat
the Y valve motors over and BANG!
I have bled the radiators lots and lots
bled the HW system via the Braukmann vent
but with no success...