Hi,
I think you will find that what you need (to watch s-video from your telewest box) is a video standards converter RGB > S-Video.
The electric signals which are used to send the picture for each of tehse formats are very different. Hence when the pj is expecting S-Video and gets (probably composite) you get the b&w pic with interference. You are sending the wrong signal to the projector.
I doubt the little box from maplin does any conversion. I think it is intended to be used with TV's where a one scart socket can be switched for use as an S-Video input or a Composite input. You plug this into the scart on your TV and you have input / outputs for composite, audio L&R and S Video. The important point here is that the box can only output what the equipment can - it just provides the correct plug.
I guess you are trying to run RGB on the cable box and then output this as S-Video to your amp. If so goto
www.rgbtosvideo.com for what you actually do need
HTH
Tim