I went down a wikipedia rabbit hole and came across this article about the Luhn checksum used for bank accounts and credit/debit card numbers (I've always wondered what the checksum method for card numbers was)
I found the article a bit confusing at times but managed to use the pseudocode and convert it into Snap!
https://snap.berkeley.edu/snap/snap.html#present:Username=cymplecy&ProjectName=LuhnChecksum&editMode
Works for me (in UK) with my debit card and my bank account (You have to prepend your account number with your sort code)