mackem wrote:
I have a 15mm pipe supplying my upstairs mains water for the bathroom and
loft storage tank.
Downstairs there is a stop cock in this pipe and ideally I would like to
have a drain valve immediately above the stop cock to allow me to drain all
the cold water pipework upstairs.

Pipework, or all water?
If all water, you're going to need either a seperate pipe, or a anti
backflow valve just before the existing shutoff valve (in case of low
mains pressure), an anti-backflow valve mounted the other way in the tank,
so it'll let water flow out, connected to the existing pipe, a tee and
a drain-valve just above the existing stopcock.
Otherwise, a T above the existing valve, followed by a valve will work,
sort-of.
(as there is no means of admitting air, so the pipe may take a while to
drain.)

Would appreciate any advice on parts I could use for this.