How to pay Notice of Assessment (from Venture Capital Reliefs Team)?

I have received a Notice of Assessment from HMRC (from the Venture Capital Reliefs Team). Despite being 6 pages in length, it doesn't explain how I can pay other than giving a Payment Reference (14 characters starting XX)which I need to quote. When I tried paying this amount via my online Self Assessment account, it doesn't give me the option to quote the required Payment Reference but is hard-coded to quote my UTR. Can anyone point me to where I can pay this bill?

Hi Excelsior,
You can pay HMRC using the methods advised at:
Pay taxes, penalties or enquiry settlements
Thank you.