How to work out interest on web saver account, (bank of scotland)?

I am due to be paid interest on my variable web saver account with the bank of scotland (5.15%) how is this calculated? i have allready emailed the bank but as usual you never get a straight answer, my balance has fluctulated over the year.thank yousorry the interest is actually 5.40%pa
Answer
I would guess that they average the account size by taking the lowest balance each day and then averaging that to to get what to apply the interest rate to.