Hi Lucy,
Thank you for your question.
I would advise you that any unused Finance Costs( mortgage interest payments) can be carried forward if they are not fully utilised in an earlier tax year.
Please refer to the guidance held here:
Tax relief for residential landlords: how it's worked out
You should keep a running total of the unused residential finance costs in either Box 45 of a paper tax return or the equivalent box on an online tax return.
Thank you.