You're talking about the box on the current form, in the charges and deductions section. This seems to have replaced the previous Mortgage Interest box, which was indeed available up till 2006 in the Personal Tax Credits section. If you checked that checkbox, it allowed you to enter the mortgage interest paid, and assigned the credit accordingly.
In the help section for this part of the form, this is what the instructions said:
"Enter the net amount of interest paid on a loan used for the purchase, repair, development or improvement of your main residence where Tax Relief at Source has not been granted. Note where Tax Relief at source has been granted on another loan(s) then ensure the amount of interest claimed here plus the interest on the Tax Relief at Source loans(s) does not exceed the allowable limits."
No mention of whether the loan was secured or not. So obviously mortgage interest could be claimed here.