Rick Dipper wrote:

I have worked out the U values of the insulation I intend to use in my
cavity walls, I have 2 solutions

1) Blocks -celotex-rockwall = 0.224 W/m2K
2) Blocks -2 * rockwall = 0.255 W/m2K

I did not take account of the external wall, plaster, mortar or wall
ties in working out these figures. The external walls are 600mm stone
walls.

Normally I would go for option 1, but its arround twice the cost of
option 2, a whole lot more difficult to install (I am building the
inner wall second) and there is not a big difference in the insulation
values.

Any ideas on how I work out if its worth it ?

Is it as simple as working out the temperature difference, the surface
area of the wall, to work out the loss via the wall, and then mutiply
by the cost of a watt of heat ?

Yes.

If you've correctly calculated the U value.
You do know that you can't add U values to get the value of the composite
wall, but have to add the R values to get a composite R value, then
convert that to a U value (R=1/U)


My thoughts right now are to save the money, and use it to put extra
into the roofspace.

U value gives you the number of watts per square meter at a 1K (1C)
difference.

To get the number of average watts you'll need to overcome the heat lost
from the walls, you take the average temperature difference (in C), multiply
by the area of the walls, multiply this by the U value, and you'r there.