Like others, I suspect that it's a scam.
However . . .
A few years ago I got a call which allegedly came from Santander, regarding suspicious withdrawals from my account. The caller wanted to run some security checks in order to ensure that he was speaking to the correct person. Like you, Tamaris, I was VERY suspicious but it eventually turned out that he really was from Santander and that three unauthorised withdrawals, totalling £800, had been made from my account. (I eventually got it back).
So, now that we seem to have established that the phone number may well have been genuine, I suggest calling 0845 300 4278 (which is DEFINITELY genuine!) to see if there really is a problem