Leave your DVD as it is. Connect a scart cable from the output scart socket in the Sky+ to one of the available scart sockets on the scart switch. Do the same with the Telewest box. Connect the Scart switch direct to the remaining scart socket in your TV.
You'll get a scart switch (sometimes called a splitter) from places like Maplins and Argos and (puts on voiceover voice) and most good electrical retailers. Or have a quick google. I'd advise against the very cheap ones as they can be very poor quality.
There's a good link here for different connections but there isn't one that gives your set up. It may give you an idea of what is required though.
http://www.flattelly.co.uk/shop/connectivity.php#13