OTOH Jacob's calculations are on heat loss and not including getting rid of
unwanted heat gain. So how much would I save, Jacob, on my summer air/con
power bills if I double glazed (which I won't). Figure on air/con being
'on' for a total of about 5 months a year [June - September plus bits and
pieces in other months.

Come to think - summer heat gain would be based on difference between
reasonable room temp say 65F and high summer air temp say 85F - 20deg
F difference. Not a lot compared to winter frost, say - 5F; heat loss
based on temp difference of 70 deg F. Hence DG contribution summer
saving less than a 3rd of winter saving on any particular day with
these temperatures. It would depend on the frequency of high/low temps
and the efficiency of cooling compared to heating. I'd guess cooling
less efficient than heating but I don't know.
Opening windows a cheaper and more pleasant option.

cheers Jacob