I don't have access to the Basic PAYE Tools HMRC insists on, but calculating manually:
Assuming basic salary £3400 per month, annual tax bill is £6272, and you would expect to have deductions of £523 per month.
A bonus £2000 would bring the total tax bill to £6942.
If the bonus is paid in Month 1, tax in Month 1 is £1312 and £1893 Month 2,
i.e. £512 deducted Month 2.
Each remaining month £512 would be deducted, around £11 less than "expected amount".
Explanation:
1) tax deductions £1312 plus 11 x £512 comes out to £6944.
OR,
2) the "extra" tax due on the bonus is £670; £790 extra is collected month 1, and "expected tax" is reduced by £121 over the remaining months.