Hello and welcome to the site. Thank you for your question.
You say the rental income is little more than the mortgage payments but with maintenance and insurance charges there is overall a small loss. Interest on loan is allowable as an expense but repayment of principal is not allowable as an expense because it is repaying of capital.
You should register for self assessment (if you are already not registered) and file the tax returns for past years.You say you have not filed a tax return for the past three years showing rental income but did you file a tax return otherwise prior to it.
If you are not registered for self assessment, you can do it online and the link is here
You should be mindful, you may be charged fixed penalty for late submission of tax returns to regularise your income and disclose income from other sources.
The fact there is no profit from rental income, you would not be subject to penalty and interest on late payment of tax.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.