Sorry for answering you by asking a question! Most TVs and DVD players have 2 SCART sockets each, so assuming this to be the case with yours, this is what I suggest you do:
(names of sockets may be slightly different)
Connect your DVD player's AV1 (or RGB out) socket to your TV's EXT 1 (RGB) socket. (This ensures you view your DVDs in best quality)
Connect your NTL set top box to your DVD player's AV2 socket. (This is known as 'daisy-chaining')
Connect your PSOne output to your TV's EXT 2 socket.
If, however, your TV has only one SCART socket, you could either obtain and pug into it a '2 into 1' SCART adapter.
And if your DVD player has only one SCART socket also, you should obtain a '3 into 1' SCART adapter, plug that into your TV's socket, and connect all three of your input devices to that. (Maplin sells these accessories).