Now this is tricky. If interest is paid annually on the account then it is classed as income for the year it was paid.
So if the interest was due to be paid after 05/04/08, it is income for the current tax year and if subject to income tax.
If the interest was paid monthly, then of course only the interest on months after 05/04/08 would be taxed, the prior months being tax free.
It is just bad timing, I'm afraid. Had it matured on 04/04/08 then of course it would have been tax free.