This is what the High Court rules say - any application to challenge a planning decision has to be commenced within 6 weeks of the action complained of.   However, this overlooks the general case management powers of the court, which allow a judge to "extend or shorten the time for compliance with any rule, practice direction or court order (even if an application for extension is made after the time for compliance has expired)".  And this power to extend the time limit applies to the 6 week judicial review commencement date too.

In normal circumstances you would expect the court to apply the 6 week rule and not allow extensions of time, as it is in the public interest that planning decisions can be relied upon.  There will, however, be the odd exception and the case summarised in this article is a case in point - where on the facts, a judicial review brought 6 years after the event was allowed.

These sorts of case are the exception, and where "subject to planning" deals are being done, there is no practical alternative to treating a permission as unchallengeable if no judicial review is brought within 6 weeks.   But maybe this reminds us lawyers to advise our clients that while the rules say 6 weeks, the court always has the power, where appropriate, to bend those rules.