Found a feed in the sockets ring so tapped I into that pics coming in my house thread. Fitted downlights too

Not for one socket no. If its just for a dis box then it would be ok on the lighting circuit. Just make sure you chop into a mains cable and not a switch. Would be better on sockets though if you can spur off easy enough

If I take one off an existing socket down the back of the walls do I have to put a fused spur in. Bearing in mind the appliance will have its own fused plug

It's just for a TV amp distributor

No. Doubtful you'll have any ringmain cables up there. It will all be lighting. By all means put a socket on the lighting circuit if you must but you'll only have 6amp to play with.

Yes.

I wish to put a power socket in loft. Can I just find an existing power cable up there and cut into it putting the socket in line to continue the ring...