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We're a corner house with square bays on two sides - I think we had quotes that were about the same but I think I'd need a scratchcard win before we could go there.
Nice-looking sash windows like yours would be a non-negotiable though.
tbc-
Ours needed totally replaced and we had factored that in when buying (along with everything else, the shocking windows meant we could afford the purchase price). The extra £4k has kind of been lost amongst everything else. We went for sash just because I think they look nicer, not out of any deference to the period nature of the house. Ours is the end of a terrace of four mirrored/identical houses. Next door still has the original sashes, but they have been maintained. The other two houses have really ugly uPVC. Not just because they are uPVC, just really weird choice of pattern and opening panes.
Even in these couple of days, the stats for our Tado valves show the heating coming on much less frequently to keep the same temp.
stevo_com
Everest weren't the cheapest, true. These were £14k all in (including install, scaffold etc). The non sash option would have been about £10k. But then these are all about 6ft tall with the narrowest one over 2ft wide. There were 11 of them and most of them needed the old sash box and mechanism removed which was an extra cost.