You should not be doing any forms to register for sefl assessment if you are already in self assessment. So presuming self assessments are already in the mix (being issued) then don't do sa1 or online registration for sole trader or online registration for self assessment.
To submit your tax return online when you are already within self assessmnet you need to link up you existing `self assessment record" to your government gateway or personal tax account that exists. If you dont have either a perosnla tax account of governmrnt gateway OR it doesnt let you add perosnal tax to your governmnet gateway.
Here is the link to sign up to your perosnal tax account which i am guesing is what needs to be done here.
The final fly in the mointment is that some webrowsers and hmrc don't get along yet - if you are following the link to signup and it is trying to sign you in then try a different browser / try private mode or try cleaning all your cookies (not your cookie jar cookies !) - or ring the on;line services helpdesk - when i say ring webchat is probably the best plan if there are any issues.
Once setup then you may need to still link up the self assessment record.
Thanks for replying! It does appear that he is going to have to go through GOV.UK Verify, not something we feel too comfortable doing but, without a direct link to a form to register for online self assessment if you already have been submitting paper forms, there may not be any other option.
everything is being moved over to the direct gov website nowadays - so i think you will have to follow whatever needs to be done in that rgeard - i do know a few bods who managed to getb their whole perosnal tax accoiunt up and runnig with ability to view all years stamp contributions in less than half an hour - so when it does work there is some nifty stuff there