HMRC - Register as Self Employed

[The Bloop]

It is a bit of a nightmare to navigate, but you'll need the gateway ID and my advice is to write it down somewhere as it's a pain to find again.
I may be wrong but I think there's something called a UTR that you'll also need, but get given by them, again make sure you write it down somewhere.
When you don't need to do a self assessment again make sure you update them or they will send you a fine for not filing your returns.

The Gateway ID number (12 digits I think) is what you create first to enable you to access your personal tax records. You'll then get a 10 digit SA-UTR (Unique Tax Reference) when you register for self assessment. These are the 2 important numbers to remember.
Each time you log onto your gateway you'll be sent (to your mobile if you want) a unique 6 digit code as a security check.

[A Quiet Monkfish]

The Gateway ID number (12 digits I think) is what you create first to enable you to access your personal tax records. You'll then get a 10 digit SA-UTR (Unique Tax Reference) when you register for self assessment. These are the 2 important numbers to remember.
Each time you log onto your gateway you'll be sent (to your mobile if you want) a unique 6 digit code as a security check.

I gave up with it years ago and now get my accountant to do it. Sounds lazy, but reading this reminded me of how I used to get so utterly fed up with all the numbers/codes/passwords. Used to drive me mad. All done now for an extra £30 !!