I have just received an email from a friend who has had an email purporting to be from HMRC, offering a tax refund and giving you your supposed tax file number, that only lasts for 24 hrs, and a link to follow.
This is a scam - you are likely to be asked for bank details into which the refund will be paid, do not be taken in by it.
here is a link to the HMRC website about the matter.
Sorry if this has been done before, I did do a search to check and nothing came up.