Living off the mains gas grid, heating with LPG is expensive. Twice
the price, if I've read this table correctly: http://www.nottenergy.com/energy-costs-comparison3

We've done all we can with insulation, but the current double glazing
on the windows is old and incredibly thin (4mm or something).Last
winter, the snow remained on the roof all the way through until the
melt, but the velux windows were melted off in about an hour!

We had a quote from a local manufacturer, and actually, it's not as
bad as I thought it would be.
This is for replacing the glass into the current, sound, traditionally
shaped wood frames, including curves at the tops (so glass needs to be
shaped)
I've turned his paper "pick and mix" quote and u-values into a
spreadsheet for my own info - does all this look right?

https://spreadsheets.google.com/ccc?...Sz7wL&hl=en_GB

In particular, note the decreases in u-values regarding the filler gas
vs the increase in price. Is it actually worth going for Krypton?

We're hoping to get a fair bit of passive solar gain through the south
facing patio doors and windows.

The company come well recommended locally and by neighbours, and to be
honest, the bloke doing the quote was so pleasant and un-pushy, if you
think this looks fair I don't think I'll even bother with a second
quote.
The last time I got a glazing quote at my old house, it was a
thoroughly unpleasant and pushy experience and came to a stupidly high
price so I didn't bother.

Anyway, whaddya reckon? Do my sums make sense? Googling around here
and elsewhere seems to show that having a gas fill is definitely
noticeably worth it, but would you bother with the krypton?

Thank you.