Hi Andrew, yes I have the manuals and it does give a part number for the sensor electrode but not the gasket. I think the leak may be in one of the pipes inside the heat exchanger and if I can find out where to get a gasket and replace the faulty one, what will happen ? If the water has nowhere to escape to and the leak is in an internal pipe the whole heat exchanger may fill up with water until it puts out the flames or renders the ignition electrodes ineffective.

I might just opt for a cheaper repacement normal combi boiler, the condensing type may be cheaper to run but the pay back time must be some years. Had thought of a condensing combi boiler from B and Q until I read the reviews on this web site.

Will keep looking at e-bay for now.

Thanks for your help, Dan.