Work with conductivities, which is the reciprocal of the uValues. Then you
can multiply them by the area, add them up and take the reciprocal again to
get the final value.

Finally, are you sure you can't get anything under the joists? Cold bridging
has deleterious effects in addition to the overall energy loss, which is all
the BCO will be interested in.

In particular, with cold bridging, you will get cold lines on the
plasterboard which will discolour and attract mould. You would be far
better, if at all possible, to install even a little insulation underneath,
even if it is only 10mm or 12mm to eliminate this possibility and provide a
uniform warm surface to the plaster.

Replacing rockwool with celotex is unlikely to be a bad idea, even with the
underboarding. The building regs minimum is a minimum. It would be to your
own benefit to improve on it.

What about conductivities in series ?
I guess these operate in the reciprocal sense to the reciprocal rule,
if that makes sense.

I also need calcs for the roof, so I am seeing a structural engineer
anyway next
week. I will see if he can get the roof thinner and allow for the
celotex.
I would still need to work out the thickness of celotex to get the
U-value acceptable.
By the way, the reason for the roof thickness fuss, is that I need the
pitch over
12.5 degrees for the tiles, I need headroom at the eves and I have a
mono-pitch roof on the party wall to line up with. And ... I'm keeping
the roof open nearly to the apex.

Simon.