Evo
The best advice I had was "Don't even bother trying with the Inland Revenue online submission."
I did try, nevertheless, and it proved to be a waste of a half an hour or more of good core lifetime.
Do the paper one, and post it - you've got to get it all together on paper anyway, to be sure you're happy with all the details.
Anyway, as soon as you need any of the "extra forms", you find they don't have them yet so it's back to the paper form.
Meanwhile - I've been able to get into the AIS site every time I've tried this past two days!