Tax underpaid despite updating my earnings monthly- where do I stand?

[Stuckinastorm]

Hi today I had a letter from hmrc- I owe over £400 from last year in paye tax.
I rang them today.... It's based on the p60 my employer sent them. Spoke to my employer- who sent hmrc my earnings every month rather than just at end of year. Hmrc changed my tax code during the year, after 1 change I got over £300 back in tax via my payslip (I assumed this was ok as code had just changed). My employer can't see how if have underpaid when hmrc were constantly updated and all codes correctly applied.
Where do I stand when it's obviously hmrc's mistake?

It's your responsibility as the taxpayer to pay the right amount of tax. You can, in exceptional circumstances, argue that you could not reasonably have expected to know that you had underpaid. However, experience leads me to say that you should plan to pay back the money owed rather than expect HMRC to just forget all about it. Can it be done through your tax-code or do they want a cheque?

They want a cheque.
The fact they knew monthly what I was getting and they changed my tax code once or twice through the year and I spoke to them I the phone about my earnings part way through the year I thought it was right.
In the future how will I know it's right please? As their tax codes were obviously wrong compared with what they wanted

Can they add it into your tax code and deduct it in instalments rather than all at once? They will normally allow payment arrangements. Otherwise get a calculator and work out yourself what the tax should have been.

or use this
tools.hmrc.gov.uk/hmrctaxcalculator/screen/Personal+Tax+Calculator/en-GB/summary?user=guest

Having been burned with serious tax code errors in the past - thousands!! - my personal solution has been to file a self assessment return every year. That way I know I'm all square

Your employer MUST have made an error on the RTI submissions.
Ask for a copy of your annual summary, send it to HMRC and find out where they went wrong.