Tax code changed 3X - now I owe them £100?

OP - Dazed & Confused is right. He's also right you will need to look at your P45's + p60 + P11d figures.

To truly know if you have paid the right amount of tax, we would need to know a lot more information i.e. other sources of income such as interest earnt from bank account savings, dividend income, any allowable tax deductions etc.... (but lets not get that complicated).

If we keep it really, really simple and assume you are only a 20% tax payer and also only look at earnings + benefit in kind less tax already paid, you might want to do this:

Add up all earnings (can be found on your P45's and P60). To this add the total value of benefit in kind (figure comes from P11d). for example

Total earnings of £42,567 + £2,237 for BiK = £44,804

Now take £44,804 and take away your tax free allowance of £11,850 thus giving taxable income of £32,954

£32,954 is within the 20% tax bracket thus you would owe £6,591 in income tax.

Now go back to all the P45's and P60 and add up how much tax you actually paid and now you should know if you have over paid or under paid or paid exactually the right amount.

Earnings = £42,567
BIK = £2,237

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TOTAL = £44,804
Less Tax free = £11,850

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Taxable = £32,954

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Tax payable = £6,591 (£32,954 x 20%)
Less Tax paid=£xxxxx (total you have actually paid on p45's + p60)