Private pension providers now deduct the tax at source. I recall there was change a few years ago.
If you were receiving gross pension contributions prior to 2010 then HMRC will say it was your responsibility to ensure you were paying any tax due.
When someone gets private pensions as well as a state pension the total income often means some tax is due.
I don't know whether HMRC will allow you to spread the cost through your tax code but I would choose that option if it's available, unless you have enough spare savings to cover it and would prefer to clear the problem quickly