Depends when you leave and what tax paid already.

If you were an employee and PAYE for example you'd typically have a monthly income tax deduction based on seeing out the full tax year and if not seeing out the full year then you'd get some back. I left in the month of September and got some of what I'd paid back

But I guess your other answer suggests self employment.

If it is under £10,000 they're unlikely to go too far at the moment. I know because I know someone who works for HMRC, due to cut backs and staff numbers anything under £10k gets put on the 'someday' pile.

In the future they may chase you for it and as an individual they're more likely to.

As much as I hate to pay tax, I'm self employed and I pay it. Its the law and its the morally correct thing to do. You lived in the country paid for by taxes and therefore you should handle your tax affairs.