On Thursday, March 6, 2014 2:08:45 AM UTC, wrote:

I'd lay the cables on top of the plasterboard.

That does seem to make sense. Indeed now that I've asked the question in public I can't help feel it's blindingly obvious!

Presumably recessed downlighters would be out of the question
Hopefully you'll come up with a better idea. Not all fashions are good.

He he. Quite. Bizarrely, I've often felt in other people's houses that that whilst I like the look of them in a kitchen environment they don't seem to always function particularly well, especially given I always end up sitting right underneath one of them feeling like I'm on Mastermind. There's no reason to believe my implementation would be any better so I'll scrap that idea.

On a related note, what are the benefits of adopting an insulation
between AND under the rafter approach?

Better insulation. When only between rafters, the rafters still conduct thermally. OTOH you can stick cheap squishy stuff between them.

I am still getting to grips with calcuating U-values so I might give that alternative a thought. I'd always assumed PIR boards were generally always better performers than mineral wool etc for a given thickness though?

Mathew