>>>the TPS is a voluntary code of practice isn't it ?
No it isn't. The Privacy and Electronic (EC Directive) Regulations 2003 place a legal obligation upon businesses to comply with the provisions of the register.
This link is about still receiving unsolicited calls after registering with TPS:
http://www.tpsonline.org.uk/tps/stillgetcalls.html
The complaints procedure is here:
https://complaints.tpsonline.org.uk/consumer
As a criminal offence (as defined by the Protection from Harassment Act 1997) a successful prosecution can only be brought when it can be shown that the actions undertaken by the defendant were both
(a) repeated ; and
(b) targeted against a specific individual.
It's likely that a civil court would use similar guidelines, so the chances of successful civil action against a caller for harassment would appear to be extremely slim.