mortgage interest relief for tax return

Tax Planning and Financial

In 2011 I remortgaged and added an extra £30k, on an interest only basis, to my residential/home mortgage to buy my first rental property. The mortgage statement showed both interest only and repayment costs and (I seem to think)I used the £30k interest payments against my tax for 2011-12.
Last year I remortgaged my own house again but I could only take out a repayment mortgage, as any interest only aspects were not available.
For this tax return I'm at a loss as to how to work out the interest paid on the initial £30k (which will now be slightly less than that as it's being repaid as part of the whole sum).
Can anyone offer any advice as to how to calculate this, and, if indeed I'm correct in trying to claim this as an allowance for the dreaded tax return?