Just caught up with this thread and noticed the reply from Fred which implies he's in Sheffield.
I've had three Sheffield plumbers round to quote for a straightforward replacement of a gravity hot water system by a combi. The first quoted £3800 all in for a condensing Vaillant. The second quoted £1230 all in for a non-condensing Ferroli, but he wasn't Corgi registered. The third told me that the condensing boiler regulations coming in next April have been dropped, so I didn't bother him for a quote. A fourth didn't bother to get back to me after leaving a message on his answerphone.
Fred, would you be able to tell me who in Sheffield you got quotes from, who got the job, and whether you were satisfied?
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Rob