Pay HMRC for "paying too little tax"

I have had a letter from HMRC saying I owe money as I paid too little tax in the last tax year. The letter says HMRC will retrieve the money by increasing tax taken from my pension in the next tax year. However the leaflet enclosed (on P800 tax calculation) mentions this but also says that I can pay online, I'd rather just pay now but, if I do so, will it mean HMRC will NOT alter my tax code next year? And any advice on paying online please.

Hi,
If you do not pay the payment in full, it will be collected via your tax code in the following tax year.
If you chose to pay the full amount online as stated in the letter, then HMRC would remove the underpayment from your next years' code upon allocation of the payment received. 
Thank you.

Hi My mother's english isn't very good today she received a letter saying she owes £50.40 to HMRC as she paid too little tax. Does she need pay it once or will it get deducted via her salary as in 12 months instaments?

Hi,
Unfortunately, it is not possible to check individual tax records through this forum, if you have a question which relates to your personal tax situation or tax code please contact our Income Tax team.
Income Tax: general enquiries
Thank you.