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Hi Gasman, I saw your explanation re the pipe size, but was waiting for the quote to come through before replying (which came only yesterday!).
The guy who thought we wouldn't need to replace the pipe has quoted a Vaillant ecotec 428, new cylinder, new pump, room thermo and a powerflush 2yr parts labour at circa 3200.
Were swaying towards going with the guy in the first post, cheapest and seems to be doing more for the cash.
We also thought if this needs doing, then get it done now before we do the house up (moved semi recently).