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Hi Etch. In the three weeks up to the boiler installation on 14th January I was using around 1 cubic metre per day; since the new boiler it's 1.6, and, as I said in my original post, I've been using it in exactly the same way, - putting it on for a set amount of time each day (about an hour), with same amount of hot water usage (very little).
Contacted Worcester Bosch who said the consumption seems correct, but that does not explain why the old one was using so much less. My next-door neighbour, who is in her 80s and has her heating coming on at 6:30am for a total of around nine/ten hours a day averages 5.5 cubic metres - which sounds about right to me.
I reckon I've come to the end of the road on this to be honest. Not what I was expecting when I forked out for an 'A' rated boiler. Thank goodness, though, that my gas and electric tariffs are fixed till October!