Mortgage interest on self assessment tax return

I understand that it’s no longer possible to claim tax relief on mortgage interest, but how do you claim the 20%? I’ve put my interest total in section 44 but I don’t think it’s come off my total tax bill.
Any pointers? Thanks folks!

It will show on SA305 I think

Box 44 is where you put it. Have a look at the full tax calculation. There should be a line near the bottom that says “Minus relief for finance costs” which shows the 20% allowance.