If the payment that was due to you included interest, then no tax would be payable on the basic repayment but it would be payable on the interest. However, in that case, the pension provider should have already deducted the tax and paid it directly to HMRC (giving you a tax deduction certificate).
It should be fairly easy to check the statement to see if and how much tax has been took off. If intrest is taxable at sauce for these, just like on PPI, then you should see it but you could claim some back dependant on how much other intrest you get as your allowed up to £1000 a think it is a year