Kiltrash

I've made a payment to HMRC for tax owed by a Trust, but my payment is shown as 'unallocated' and the tax owed by the trust is still shown as being due for payment. My questions are: How do I get HMRC to use my payment to clear the tax owed? Is there an extra information that needs to be stated when paying the tax bill? (I simply used the 'How to pay' link on the Current (Tax) Position page for the Trust. Thanks