How can I fix the gas valve on my britony 2 t mutipoint?

Although the ignition light stays on (my Britony 2 T multipoint) will not ignite even though the diaphragm has been changed. The engineer tells me that the gas valve is jammed and corroded and although he has tried to free it, it is a sealed unit and the unit has to be changed completly at a cost of £260 + labour, is this necessary?