Hah, BCO is wrong. He probably looked at the cold bridging and got
scared
I calculated for my case and the one from the knauf website.
My figure for the case on the website came up with 0.18 which matched
their figure, so sums OK.
My figure for my case came up with 0.199. Less than 0.2.
And that's without including the plasterboard or boundary layer.
So, I can prove it if I want. If he disagrees, he will have to show me
his calcs !

But, I take on board the cold lines factor. But, 225mm of wood is a
relatively good
insulator I would have thought, so not sure this would be a problem in
practice.

But I need to keep on good terms, I may have to confront him with
legalities if
he wants me to pay for the electrical inspection. They want a note in
the spec
that "a test certificate to BSxxxx will be given to building control".
I'll have to be
careful how I word that bit.

Simon.