There is the lighting circuit up there, and conveniently near to where I
need the socket, there is a brown juntion box which just has 1 twin+earth
in, and one the same out. Perhaps someone used this to join the cables?

Fine, if it is permanent live.

Anyway. Can I just connect a length of twin+earth into here, then connect
the other end into a 5A single socket.

Yes.

I'll have to cut the flex from the TV amp and fit a 5A fuse with the
rounded
pins to fit the socket.

The round pin plugs are not usually fused. The fuse will be the 5/6A
fuse/MCB at the consumer unit.

Does this sound OK?

Yes.

Ideally i'd like a double socket up there because for a while whilst i'm
installing the aerial stuff i'd maybe like a spare "normal pin" socket
available to charge a small power tool, but these perhaps could be
misinterpreted as a "proper" socket in the future and someone might
overload
the lighting circuit?

Indeed. I would not do this. Just use an extension lead from downstairs if
you can't extend or spur from the ring.

Christian.