This is a scam that has been going around for the last couple of years. You receive an email supposedly from HM Revenue and Customs indicated that a tax refund is due, and all they need are your bank details to repay you. Do not reply, it’s a scam.
Firstly, HM Revenue and Customs will not email you directly, and secondly, if there is a credit on your account, they will not offer to simply pay it back. Anyone who has received a tax refund recently may appreciate how hard it can be to get your money repaid to you.
Further details are available here.