Looking at the joints, the manufacturers recommend sleeves are used to
support the pipe internally, I presume to stop it being crushed. Bearing
in
mind its only 15mm pipe, will these sleeves restrict to any great degree
the
flow rate?

No, they're very thin, provided you get the right type. (I use Hep2O for
this).

To join the pipe to the existing copper I was considering the use of a
full
bore valve, such as a stopcock.

Use a quarter turn lever ball valve. Compression are cheaper than pushfit.

i.e.

http://www.screwfix.com/app/sfd/cat/...09371&id=18343

Christian.