ive had this problem.its the filter.a silver box fitted at the back of the boiler.eventualy tracked the problem down,and had it flushed out.it was ok for a couple of weeks,then went again.we then had a new one fitted and that was ok for a couple of months,and has just happened again.so now we will have to find out why this keeps happening.ime 100% sure this is your problem.it looks like the bottom of a burnt sausepan when cleaned out.!.hope this helps.
nigel.
May 2010
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