Dim boy

I currently live overseas and have had rental income in the years 22/23 and 23/24, but because it was considerably under my personal allowance, and that I paid tax on it in my current country of residence, I did not realise I needed to declare the income to HMRC. Can anyone advise as to what the consequences are likely to be? I don't believe that I owe HMRC any tax (as I have a long-term illness and no other tax liabilities or income), but I'm concerned by some of the penalty figures that I've seen mentioned elsewhere. I want to get it sorted, but don't know if I should contact an accountant or the HMRC first. Any help greatly appreciated.