>The tax office sorts that out for you as soon as you register as being self-employed<
In theory, yes, in my case it took 5 months of phone calls and letters to make them realise that I should be paying Self Employed NI contributions, as they didn't have me registered as self employed, then they informed me that as my SE profit was under the compulsory threshold for NI, I didn't need to pay it after all!! It then took nearly a year to get hold of an exemption certificate from them :-( Unfortunately, as in many large organisations, there are some super efficient staff, and some who seem to forget what they're supposed to do next as soon as they've put the phone down! :-)
HMRC run some quite useful free courses on how to keep accounts, fill in your SA form etc. I try to keep approx 30% of my profit to one side in a savings account to pay my tax bill at the end of each year, so I have the money there when it's due. I don't use an accountant, mainly because my accounts are very simple and I'm use to doing loads of complex form filling in my 'normal' employed job, so completing a SA form seems quite straightforward!! If you're not used to dealing with things like tax deductable expenses / capital purchase deductions etc, I would recommend an accountant though, if only to 'walk' you though your accounts for the first year.
Claire