My Vokera 36he fine at heating but hot water cold?

Exellent for heating but when asked for DHW (domestic hot water) it cut the boiler out displaying fault 79 (temp too hot leaving boiler) irrespective of the boiler setting (economy, pre-heat, hot water only) or the tempreture setting from 40 to 70 degrees for dhw (norm 46)

this boiler was purchased a year ago but only fitted a few months ago and only switched on two weeks ago due to planning hold ups so it is virtually brand new.

can some kind soul save me please cos the wife has told me the next time shes in the bath impersonating a polar bear i'll be on a park-bench impersonating a homeless person