HMRC sent letter to old employer - now lost in post

I have not worked for my previous employer for a couple of years, however when they were recently sold I had shares which were cashed in (this has put me in the wrong tax code, but this query is not about that). Subsequent to the sale, a letter was sent to my previous employer which was addressed to me. My previous employer posted it to my home address a month ago (as I don't work there anymore), but I haven't received it and don't know what was on it. Is there any way of getting a copy of this letter sent to me directly? Or is there a digital copy I can have access to? I am concerned that there is important information that I am missing.

Hi,
We are unable to access individual records on this forum, please contact our Income Tax team for further assistance:
Income Tax: general enquiries
On this page you will also see a link to personal tax account which may be the quickest way to review this information.
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Thank you.