I think there is a lot of confusion on this thread between the rules for tax credits and universal credit.
For working tax credit - new rules from 6 April 2015 require you to show that your business is on a commercial basis, with a view to realising a profit and is organised and regular. There is no minimum hours requirement - but HMRC will select people to check if they are earning less than the hours required x NMW. However they then have to apply the legal test I just outlined. The hours x NMW is just a way for them to select people for further checks and is not part of the test.
For universal credit - if you are in the all work requirements group, after the first year of a new business you will be required to earn at least 35 hours x NMW (unless your threshold is set lower). If you do not, then you will be treated as if you are and have your UC worked out on that figure.
However, UC is only just being rolled out and as someone posted, currently self-employed people cannot make claims for it. The only exceptions are in two digital areas (Sutton) where you can no longer claim tax credits and if you become a couple with someone on UC or if you are unemployed and claiming UC and then start self-employment.
Cleo