There is an additional threaded nut on the outside, but no instructions
or apprant way for connecting the copper (or plastic) incoming mains to
this

Personally, I'd not use the brass adaptor or the push fit fixing. I'd use a
1/2" tap connector (usually right angle "serviceman", with built in
isolation valve), which is what toilet fill valves invariably require.

Personally, I'd use a chrome compression type to connect to chrome pipe into
the wall. I'd only switch to plastic when it wasn't in view.

http://www.screwfix.com/app/sfd/cat/...70358&id=17752

If you must use push fit:

http://www.screwfix.com/app/sfd/cat/...70414&id=12129

This doesn't have a valve built in, but is white, so would look better than
a grey system on modern sanitaryware.

Christian.