Mike Mitchell wrote:
Are Crittall's steel-framed windows of yore that much worse for
insulation than cheapo plastic-framed double-glazed units?

In terms of heating bills, how long would it take to amortize the cost
(ca. ?5,000) of replacing steel-framed windows with the uPVC variety?
I reckon it would take literally years before one had paid off the
costs through fuel savings.

How much is your heating bill?
How many square meters do your windows add up to?

What if the steel-framed windows are furnished with secondary
double-glazing? Doesn't this make them equal to uPVC?

Not quite.