A brown envelope from HMRC changes the shape of your day. The instinct is either to fire back a reply or to put it in a drawer. Both make things worse.
First, find the response date on the letter — that is the clock that matters. Then note what HMRC is actually asking: is it a check, a penalty, a request for documents, or a demand for payment? The wording tells you a lot, and the right response is very different for each.
The single biggest mistake is answering incompletely or late, because it narrows your options and can escalate a routine query into an enquiry. The second biggest is disclosing the wrong thing in the wrong way.
This is exactly what HMRC SOS is for: tell us what happened or share the letter, and you get a plain-English read on what it means, how urgent it is, and what not to do — before you respond. With a signed authority, we then deal with HMRC for you.
Have this question yourself? Talk to an adviser.