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.

Dunno - you'll have to work it out for yourself!
Can't say I've ever suffered from unwanted heat gain in Britain. I'd
open the windows and generate a bit of through draught.
The french have the answer to summer heat - high ceilings, large
windows opening inwards with slatted shutters closed on the outside -
bliss, who needs air conditioning?

cheers

Jacob