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 ?

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

Thanks
Rick