I knew immediately it was a scam, so I asked him to prove that he was calling from the bank. He tried to bluff, saying he could give me the phone number of the head office, but I interrupted saying that I'd check my account online and if there was a suspicious payment, I would call the fraud help line myself. At which point he quickly hung up.
But it occurred to me that it might be fun to play with them a little. What if I'd replied, "Oh, no! That's fine. We've just bought a second hand car off Mr Singh." What would they say then?
If I'm in the mood, I like to keep scammers talking for a while if I can because if they're talking to me, they're not cheating anyone who is vulnerable, but this is the first time we've had this particular scam, so I hadn't thought out what to say and therefore went straight on the attack. I did try to get their number by calling 1471, but of course it had been withheld, so I can't report the scam attempt.