On Friday, March 7, 2014 4:25:40 PM UTC, RobertL wrote:

But surely there has to be some insulation below the rafters as well (for
example battened-on thinsulex). You can't get the required U value just with
insulation between the rafters.

You can with 165mm Celotex and 175mm rafters (using 12.5mm plasterboard). This gives a U-value of 0.16. As mentioned in the thread though, the bigger rafters obviously cost more.

Also, as it's a kitchen you need a Part-P electrician to do the wiring

Indeed, but only as far as running the new circuits in i.e. a couple of radials for the oven/hob, and a ring for two sockets. Extending the latter, and sorting the lighting circuit, is something I'll do myself in due course.

Mathew