Thanks to everyone for the replies.
The answer seems to be that I will have to get a Pressure Relief Valve fitted on the Domestic Hot Water input side in order to help ensure that another fragile heat exchanger doesn't go - have to say I won't be buying Vokera gain.
New Heat Exchanger...
Less than a year after having this new boiler fitted the pressure rose, water started discharging on to the roof via the overflow pipe and then the system shut down. The engineer came and said the heat exchanger had gone so water from one part of the system was going into the other and causing...