Thank you for your reply and for additional information.
Based on your figures your total income in the last tax year was
(23,143+1,210) = 24,353
Personal allowance = 10,000
Taxable income (24,353-10,000) = 14,353
Tax due at 20% £2,870.60
What amount did you input for tax paid against employment income of £23,143?
If based on HMRC letter you have paid tax of £2,628.60, then you owe HMRC (2,870.60-2,628.60) £242 which is 20% of taxable benefit £1,210.12 you received.
It is likely that you did not pay enough tax whilst in employment and the amount being sought includes the underpayment.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.