I doubt they are legally owed that amount, its the crap computer system that keeps dreaming up figures.
For example, one year I came to do my return, used the password from last year, it didn't work so I phoned up and asked for a password. The one they sent didn't work, so I phoned up again and asked for a password. The one they sent didn't work, so .....
4 times they sent the wrong one, I eventually managed to get on line to do it, by which time my return was now late so they gave me a £100 fine. This is prob where the vast majority of their uncollected dosh comes from, someone saw the possibility of raising lots of extra cash by making the computer system so useless that they could hit us with fines.
This year I was trying again to get it to work, I attached the return in PDF format, as you are allowed to do, but got the error message "Warning you have attached a PDF file, but have not ticked the box to say you are attaching a PDF file". And the follow up letter I received, suggests someone is not very good on grammar or comprehension of what they wrote.
Ken