Matt - not strickly true,
notes, posters, lables on the door have no standing in law, it is upto the person knocking if they choose to take
any notice of these, also the whole cold calling idea is aimed at rogue traders, ie.. gypsies who do drives etc...
bona fide business can knock
Now fully signed up NO CALLING ZONES are a bit different - usually you have a more active commitee that will report you to the TS however a proper trading business is still allowed to knock at worst TS will call you to check that you are a trading business.
Also the NO CALLING ZONE should in the first instant talk to the company/person knocking.
It's one of these great ideas that was supposed to stop grifters/con-men ripping off vunerable householders but
is abused by the NIMBY brigade.
I recall that in one area a poppy seller was reported to TS

- but cant remember thew exact details, but again
an example where instead of people being polite and saying No thanks they want to be little Hitlers.
Darran