Sammy

I am writing on behalf of my mum who owns properties in the UK and has been told that she has been penalised for not submitting a tax return form. The gov has sent her a letter with her UTR but since she does not live or work in the UK, she does not have a NI number that allows her to fill in the form online. How can I help her in this situation? She has already been fined over £500 for late submission.