In 'The Which Book of Plumbing & Central Heating' the heat loss quoted
for single glazed wooden windows is 5 W/sq.metre/deg C and for DG
wooden or plastic windows is 2.9 ditto. This means DG saves 42% heat
loss. Similar figures are quoted elsewhere, say approx 50% as a rule,
of average 10% heat loss of whole house i.e. about 5% overall.
If you work it out on paper every other way of reducing heat loss from
simple draught measures up to replacing old boiler with modern high
efficiency boiler, will cost less than whole house DG but save more or
at least be more cost efficient.
In other words DG is the last thing you should bother with - only
after putting into place ALL the other measures especially loft
insulation - and even then is not worth it unless you have money to
burn.
You can work it out for yourself.

cheers

Jacob