We've just had our first gas bill (for heating and cooking) since the cavity
wall insulation was installed mid-December.

Last winter (Nov 07- Mar 08) - average daily usage 10 units per day
This winter (Dec 08 - Mar 09) - average daily usage 7 units per day !

I didn't expect it would make that much difference, and I'd say that this
has probably been a colder winter than last year.

I reckon that means we should pay back the (70% subsidised) cost in about 15
months.