I'm not sure what you mean by "quarterly interest charge". Are you sure it's not your quarterly fee? Unless your current account offers free banking, there may be a charge for some/all of your transactions (i.e. lodging/withdrawing money, direct debits, standing orders) in which case your bank should be able to tell you what transactions you have been charged for.
On the other hand, given that it's a small amount, maybe it's your quarterly interest, and not a charge at all!