Paying fixed penalty notices

Just paid a Fixed Penalty Notice using this link:

https://caomain.latestinfo.co.uk/met/

The form asks for a fine code letter, which does not appear on the FPN. I googled this and found the following. "N" seems to cover it.

Weird that this method of payment is not printed on the fine.

It was for riding on the pavement if you are interested, I have a little duck through near St James's Park leading on to Birdcage Walk to avoid being crushed by black cabs ferrying politicians and civil servants to their burrows.

That URL doesn't look anything like an official government site, and I get a security warning if I click on the icon in the corner.

Why wouldn't it be a direct.gov.uk address?