Didnt know I could claim "Residential property finance costs"

I was reading an article and it mentioned you can claim Residential property finance costs for the interest on a buy to let property mortgage. Such a fuss was made about mortgage interest no longer being allowed as an expense that I didnt realise you could still claim 20% in a different way! If I want to backdate this "Residential property finance costs" should I: a) use the "Unused residential property finance costs brought forward" box in my latest (2022-2023) Self Assessment or b) update the "Residential property finance costs" figure on the still amendable 2021-2022 Self Assessment? Or does it make no difference? Many thanks

Hi dazzatron3000,
You will need to submit a separate claim for the earlier years and state the amounts you are claiming - please note the splits as per:-
 Tax relief for residential landlords: how it's worked out, you will also need to confirm that you are making a claim to overpayment relief.                                                                                                                                                                                                                                                                                                                                                                                    
All claims must be in writing to us at:
Self Assessment,
HM Revenue and Customs,
BX9 1AS,
United Kingdom
tell us you are claiming overpayment relief, give full details of the claim including
the amount of tax you have overpaid
why you have paid too much tax
the tax year / accounting period
if you have already appealed the calculation
if you are claiming repayment
any supporting documents showing tax paid
be signed by the customer - even if sent by an agent
confirm the details given are 'correct and complete to the best of my knowledge and belief'
                                                                                                                                                                                                                                                                                                            The earliest year you can now apply for is 2019/20
Thank you.