Right, basically it looks like the actual regulation (rather than what's stated on the page you link to) is sufficiently wooly that it can't really be brought to hold a provider to account unfortunately. I'm not an expert, but the relevant documentation is at http://www.ofcom.org.uk/static/archive/oftel/publica...
Thanks for looking at this. This seems relevant:
18.1 The Communications Provider shall provide Number Portability as soon
as it is reasonably practicable on reasonable terms, including charges,
to any of its Subscribers who so requests.
I suppose the key wording is "as soon as it is reasonably practicable". It is in Talktalk's and Sky's interest to equate that phrase to mean "as late as Ofcom will let us". I have no idea of the status of portability of Talktalk provisioned numbers, but in the case of Sky at least, it seems they do not currently allow their provisioned numbers to be ported to WLR or Talktalk.
Since my number was provisioned by BT many years ago, I have no such issues moving my number between providers, be they WLR or LLU, so "I'm all right Jack". I do however feel sorry for those customers provisioned by Sky who are tethered to them because they cannot move their number.
I do think it's a case for Plusnet (and other CPs) to push Ofcom and/or Sky though, since a customer locked in to Sky is one less potential customer for you.
Oliver.