Does it have a certificate requiring you to certify that your tax returns are correct and that you have nothing to disclose? If so it could be a generic letter based on information received by HMRC under the "global" common reporting standards.
You would also have to check that you did not have any " requirement to correct" before 30th September 2018 (yes 2018) in respect to any arrangements that had been put in place and on which you might have previously received advice that matters did not require to be disclosed to HMRC etc etc. Basically anybody that was involved or instrumental in setting up any offshore arrangements and previously gave you advice that everything was in order could not be relied upon to give you a reasonable belief that there was for example no or minimal UK tax liability. Such advisers would be disqualified advice and you would need to get a second unrelated or uninterested party opinion to confirm or revise any previous advice and belief.
Basically I have 100k euro gifted from my father since 2012. I have declared the income since but I also have an inheritance my dad received which he then gifted to me and was brought to the UK and signed under a DOV.
So I don't know whether recent large sums have alerted hmrc but it is nothing untoward on my part.
I certainly do not have offshore schemes or anything like that.
I answer yo tour question Does it have a certificate requiring you to certify that your tax returns are correct and that you have nothing to disclose? the answer is yes this is the letter I have received.
Do they send this out even if something is not necessarily wrong?