Heat loss through windows rule of thumb is about 10%.
Double glazing halves this and so saves 5% of heating component of
fuel bills.
Typically about £500 p.a. hence saving £25 p.a. Really not worth the
bother.
Higher proportion of heat lost through windows if the house is highly
insulated but the small saving is the same i.e. larger % of smaller
bill - still not worth it.
Add in rapid obsolescence of DG units makes it very costly - they all
mist up eventually.
Better to increase loft insulation, reduce draughts, thicker curtains
etc etc. Cheaper to install new gas condensing CH boiler will save
much more.
Double glazing expensive con.

cheers

Jacob