I called them, Self Assessment can only be done via gateway.gov.uk, it's a separate system to tax.service.gov.uk (according to the person on the phone).
I've registered (again!) and now have to wait for them to send an activation code in the post.
I still can't log in to the Personal Tax account thing - which I could last night - but it sounds like that's no use in this, so I guess it doesn't really matter.
I called them, Self Assessment can only be done via gateway.gov.uk, it's a separate system to tax.service.gov.uk (according to the person on the phone).
I've registered (again!) and now have to wait for them to send an activation code in the post.
I still can't log in to the Personal Tax account thing - which I could last night - but it sounds like that's no use in this, so I guess it doesn't really matter.