No you can't change the fire front. The fire and the back boiler come as a set with the front fire mounted on supports provided by the the back boiler.
As an approved set, the back boiler and fire dicharge their fumes up the same chimney flue and use the same integral gas supply. If you alter/replace or adapt you will not be creating an 'approved appliance' and you will fall foul of the gas regulations.
Any fire you try and fit will be designed to discharge directly into the chimney flue but your back boiler wont allow that.
Remember carbon monoxide is odourless and colourless and will make you and yours lifeless!
Cledlyn, August 2008
