The message
from Broadback contains these words:

I had to smile at the "scientific" figures given on energy savings by a
well know presenter at the end of his recent series. It was like:
If you do this you will save 30%, if you do that you will save 20% and
so on. What was obvious was that each % given was the saving on your
original energy consumption, as soon as you moved to 2nd option the
saving would be lower as you would no longer be using as much energy.
Of course they also took off the biggest first. You will note that when
retailers give 2 discounts, say 20% then 10% the 10% is off the reduced
not original price.

It doesn't matter which discount you take off first.

100 100
-20 -10
80 90
-8 -18
72 72

However it doesn't necessary follow that with heating savings the 2nd
saving will operate on the reduced amount. If you get a saving of 10% by
fitting double glazing and 20% by cavity wall insulation you will have
an overall saving of 30%, not 28%. OTOH the saving claimed for turning
the thermostat down (3% per degree?) does operate at the current
insulation level and can't come anywhere in the calculation but last.

--
Roger Chapman