The recent Supreme Court decision in HMRC v Tinkler serves as a reminder that clients who wish to challenge the validity of an HMRC enquiry should do so promptly.

HMRC opened an enquiry into Mr Tinkler's 2003/04 tax return on 1 July 2005. That enquiry finally closed on 30 August 2012. It was only in 2015 - two months before the subsequent First-tier Tribunal hearing and some ten years after the enquiry was opened - that Mr Tinkler raised the point that the letter opening the enquiry had been sent to the wrong address. Mr Tinkler argued that this meant that the entire enquiry was invalid.

The Supreme Court, allowing HMRC's appeal, applied the doctrine of 'estoppel by convention' to prevent Mr Tinkler from denying that HMRC had opened a valid enquiry. This was so even though Mr Tinkler was possibly correct to say that, technically, the enquiry had not been properly opened.

Therefore, clients should always take steps at an early stage to establish whether there are any technical failings in an enquiry, and raise any issues in a timely manner. As the Tinkler case shows, it is not always wise to keep such points 'in the back pocket' to rely on later if substantive arguments fail.