Simmo

I've received a letter saying I've underpaid tax by a relatively small amount but my online account does not show this. The letter also says a copy was sent to an old accountant from 25 years ago when I was self employed. I've been employed by a company for the past 20 odd years now and the accountant is dead as far as I know. How do I verify if this is real and remove the old accountant? I'm definitely not self employed.