This remains an area of great controversy. See http://blogs.journalism.co.uk/editors/2009/10/19/pcc-and-the-third-party-issue/ for example. They frequently refuse to rule on cases if the person/people directly effected do not complain.
OfCom and the ASA have no such rule, though OfCom in particular will not automatically rule on any complaint, and have followed in the steps of its predecessor, the ITC, in condemning press-organised write-in campaigns such as that over Jerry Springer the Opera (the ITC condemned the Daily Mail's gross hypocrisy in its reportage of the Brass Eye 'Paedogeddon Special'). The June 2004 ASA ruling on a Rihanna perfume (Rogue) ad was prompted by a single third party complaint, for example.
Its questionable whether the person involved is always aware or even competent to complain, but this remains the assumption
They fear being inundated as an organisation with just £2m annual budget.
The PCC don't want to be automatically forced by pressure groups to get into dialogue/judgements
The PCC publish a list of FAQs, including this: