I've just done a

on your STB and came accross this
http://www.dvd.reviewer.co.uk/forums...&Thread=266697
I really don't know if a copy protection incompatibilty with your old VCR could be an issue. I'm assuming your VCR is old as it only has the one SCART.
I admit to being stumped. Have you tried calling Telewest to see if they have any ideas?
Certainly if you only have one SCART on the VCR you will need the STB to be powered on at when the VCR is in playback if your connections are only via SCART. i.e. playback needs the CART signal to pass via the STB to the SCART on the TV. Issue then is that the STB might interfere with the VCR's transmission.