My son has the AEI digisender from Argos (79.99) which works perfectly with the Pace 4001NC STB.You don't need any coax with this sender. You can plug the receiver unit into a VCR scart - assuming you have a VCR of course- and loop the NTL signal into the TV via RF.
The senders only transmit IR signals if the STB uses the RC5 IR format and does not as yet handle IRDA signals although a separate unit is due out shortly.
You can of course try out the AEI didisender from Argos under their 16 day money back guarantee - if it works , fine , if not take it back.
Their website appaers to be unavailable at the moment:-
www.aeisecurity.com