Surprisingly from the responses I have received here, they have ALL suggested that I replace with a close system, i.e. combi boiler.
A sealed system is NOT the same as a combi boiler.
You need to locate the writing on the outside of some of the grey plastic pipe; this will identify the make and whether it is barrier or non-barrier tube. The rate of corrosion suggests that it is more likely a leak or else it is pumping over during some of the heating season,i.e it may only pump over when the CH only is on and the TRVs have mostly shut themselvesd.