BOS Certificate of Deduction of Tax incorrect

mary

As I'm self employed, I've been collecting the annual round of Certificates of Deduction of Tax. However, when I checked the one for BOS on my High Interest Current Account, it said.
Gross £104.37
Tax £17.05
Net 87.32

When I added up the amount on each statement, the £87.32 was correct.
But if you gross it up, it should be £109.15.

First phone call was a waste of time, the guy didn't know who I should contact. Today I rang the Complaints No and the girl had to go through 3 departments before she came back to me to say they would write to me, but did point out that the interest would be different because it was added at a daily rate. Yes I said, but 20% tax is just that, 20% tax, that's what
Inland Revenue demands. So I wonder who's squirrelled away the missing £4.78. Was I the only one?