Here's a thing for you to consider.
I've just drawn out £250 from the cash machine outside the Santander branch where I hold an account. I then went straight to HSBC to pay the same notes into my account there.
The HSBC machine clicked and whirred and then spat out a receipt for £230, with a message that one suspected counterfeit note had been detected and retained. The staff in the branch confirmed that the note would be examined and that if it was indeed a phony I would lose the £20. Whoever handles it last before it is detected is deemed to be guilty. This process will take a couple of weeks.
Needless to say I have rung Santander, who claim this has never happened before. Really? Does the bank check its notes before putting them in the ATMs, I wonder? If the HSBC machine was able to detect a fake note on the way in, surely the Santander system must also have similar technology in place.
Of course, it might turn out that the machine has made a mistake (surely not!). If, not, I'm sure that the good people of Santander will reimburse me (well, Rory in the call centre said they would, which is good enough for me).
The annoying thing is that if I'd queued up at the counter and handed the money over to a human, this problem wouldn't have arisen.