Thanks Jim,
I think 10mm plastic pipe is a smart idea, drilling a 38mm diameter hole to sleeve and insulate 15mm copper pipe, then running water-proof insulation to the tap as the regs would have you do is a non starter in my humble opinion, especially with new (especially timber frame) houses and their damp-proof membranes in the walls.
I usually drill 22mm hole, sleeved 15 copper pipe, DCV, stop-tap and drain off inside the house. The problem id the customer has to drain down before winter.
Its a pity the combined isolation tap and DCV can't be T'd in as I too dislike the thought of the little copper disc causing havoc somewhere in the plumbing, I've also seen loads of the old chrome washing machine ones rusted up.
Has anyone ever come across a combined stop valve, DCV and drain off ?