Posting here for maximum visibility, obvs I'm not BU at all! I found out about this from one of those pop up ads on FB and was all 'Oh yeah another PPI type nonsense' but when I looked into it on the gov.uk website it is totally legit. We had a couple of years where DH was earning very little, so he went online and applied to transfer the maximum unused allowance to me and I got a cheque for £440 from HMRC a couple of weeks later. He said it was really easy to do as well. Well worth it for anyone married where one party is SAH or earns under the threshold.
www.gov.uk/marriage-allowance