As the responses you already have indicate, this is most definitely not a "Massive Tax Problem". While it is perhaps possible to understand that a feeling of guilt has set in, in the scheme of things this is among the smallest of problems which is easily soluble by getting a good accountant who is well versed in tax matters to sort it out. The cost is likely to be mostly accountancy fees, unless the rent received was quite substantial and over a long time. Ask the accountant to advise you on what might be due (if anything) given the amount of rent over whatever period and when the income was received, all in accordance with the overall income circumstances at the time.