So according to HMRC's online registration helpline, the registration time limit for the self-employed is 31st January following the tax year in which the business commenced. In fact, they read me the blurb that is read out to those applying by phone, which does indeed say just that.
Which would be fine, except that according to HMRC's website, the time limit is 5th October following the tax year in which the business commenced. See http://www.hmrc.gov.uk/manuals/sammanual/sam100130.htm "An individual should register as self-employed no later than six months from the end of the tax year they became self-employed."
Than there's http://www.hmrc.gov.uk/guidance/selling/tax.htm which informs us, under the heading "Time Limit for notification of [class 2] National Insurance liability": "From your trading start date you have three months to notify HMRC that you are trading, otherwise you could face a £100 penalty".
Help me out, someone: is it 9, 6, or 3 months if ALL potential late registration fines are to be avoided?