they picked up part of the Cable & Wireless / Mercury system after that collapsed
CWW certainly didn't collapse - Mercury was a subsidiary that got re-merged back into CWW, which, together with the acquisitions of Energis, Thus (and some others in the UK) became the current CWW - recently bought by Vodafone.
You are right that CWW sold off its cable TV arm to NTL (which was subsequently bought by Virgin).
Since CWW didn't collapse, no parts were available to be picked up by Plusnet, or anybody else!
Of course I could be totally misrembering,
Quite.
SD