Simple assessment demand, reference not shown

I've received a letter saying I have an overdue payment demand for a simple assessment amount. I've logged in to check the details and pay if legit and I can't find any more detail. The reference on the letter doesn't work as its supposed to begin with an x. I never got the previous letter (pa302) so how do I find this information? I can't seem to find a way to request a copy of this previous letter. I'm completely baffled on how to proceed here.

Hi,

We would need to access your record to check on the PA302. You can contact our Income Tax team at:

Income Tax: general enquiries

Thank you.