I've also had a reply from Jack Schultz on the ProntoEdit forum:

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Teresa,
I have gone to the mountain and unfortunately C & W chose a strange arrangment without a carrier for their IR scheme. It's also not possible for Pronto to recreate as it's being done within an ASIC in the remote.
The good news is that there are discussions going on to somehow get past this problem which will be dependant upon Pace and C & W's willingness to be flexible (aka responsive to end users) on the subject. It may require some sort of an external dongle or similar to provide a "normal" type of input capability.
Stay tuned as details develop. The Belgium team is "connected" on the subject.
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So if people haven't already done so, please write to Telewest or Cable & Wireless asking for a solution. The more people they are aware of that find this a problem, the more likely they are to do something about it.

Teresa