I noticed the NTL bloke using some kind of plastic plug that you
hammered the cable clip into. Looked like he was drilling a 3mm or so
hole. Haven't seem them around but no doubt electricians would be
familiar with them.

Saw them years ago, I believe they were labelled 'pin plugs'. White,
about 1" long by 1/8" diameter. At a pinch you could probably get the
same effect by drilling and then plugging with thin dowel, preferably
hardwood.

--
"Anyone who is capable of getting themselves made President should on
*no* account be allowed to do the job."
- The Hitchhiker's Guide to the Galaxy

Tower make them - "Pin Plugs", white plastic, require 5mm hole smallest
SDS drill size available! :-)

Electrical wholesalers rarely stock them for some reason - I bought my last
box from a B&Q Warehouse, shelved by the cable clips.