I could think of no reason why this should be the case, so when I got home I rang the Nectar hotline to give them a piece of my mind.
It was only when I dug the card out to read off the account number that I noticed it wasn't actually my Nectar card but one belonging to a MR C MONTANO TOBAR.
Anyone familiar with my obsessive interest in Nectar points may pause here to enjoy a moment of schadenfreude.
God knows how long I've been going around racking up points on a complete stranger's reward card, or when and how the switch occurred. So much for my powers of observation. (I suspect it might have happened at the Downham BP station - I remember seeing a stray card lying on the desk there.)
Mr Tobar, if you're reading this, I have destroyed your card as per helpline instructions. Try to look after your new one a bit better, eh?