I share ownership of a flat with another (equally) , which is rented out. However, I have bond costs and the partner does not. In filling out my tax return at the end of the applicable section it asks if I am in a partnership, and if so to give the share, or check the box to allow unequal sharing of expenses. However, checking this box does nothing.

It says somewhere else to claim as an "other deduction" but this box is shaded and will not allow me to write in it.

Is it to do with the questions at the beginning?

Any help would be appreciated