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fixed - plumber came and said a bit of the rubber inside the cistern had broken off, so that's that.
I haven't had my boiler serviced in some years and as he was upstairs fixing the cistern - I thought to myself - oh I'll ask him does he know a trustworthy boiler service man. Great minds think alike. He came downstairs and said "you know I service boilers too". Yippee - he's coming next week.
My last boiler service man was very very good (retired) and so I have the same faith in this guy. Good tradesmen are so hard to come by.