Refunded interest on Santander account

Received a bank statement from Santander with an enclosed leaflet. Part of which reads

"We've  identified that previously we haven't always calculated iinterest correctly when you used your debit card to make purchases against your Santander account.

Were very sorry for our mistake, but we've put things right for you.

There is more saying they've checked all our transactions and refunded as per the statement"

Very nice surprise on two counts, they've admitted a mistake and refunded, when we knew nothing .  But since we haven't used this as a main account since last December, thus no debit card transactions for six months I'm wondering how long it has gone on
for.

Makes you wonder where you have to draw a line under checking you are receiving the correct interest on your accounts now.  I'm pretty savvy on swooping about to get best interest rates but have never checked the bank/building society for the maths on the
interest, I do a very rough calculation. I assumed they'd have a system to do this correctly. Should I go back and check all accounts, we don't have tax deducted at source on interest this tax year, but also to check tax deducted correctly too .

Cornersteady you could make a good income in the side for those who are happy to say they can't do maths