So long as you are paying the full payment of NI then you are entitled to the full amount of MA. You also do not need to include this on your tax return.
Regarding tax credits it will be based on your 'profit' and also your H's earnings.
as an example.
Both H and I are self employed and they base it on an estimate of profits which is what you tell them or based around last years accounts. They then ask for the actual amount as per tax return and will adjust the tax credits around that. Again as an example my earnings this year will be about 2000 because I only worked from April to May and then again from March to April. I think you have to work at least 16 hours per week. My H estimated profits are 18000 therefore they work them out on income of 20,000. We get quite a lot each month and more in the babies first year.
I really worried about this in the first year and goodness knows how but I put it all away until they sent the letter after our tax returns saying that we were entitled to it all. Once I got this letter then it meant we could keep the money so I spent that money the following year and then put away the current years money - I have been doing the same each year. H business is very up and down so we might have a good year followed by a bad year so we never know if the calculation of tax credits is right and I would find it very difficult to pay it back if need be.
Hope this all helps - but with MA the key is to pay your NI - incidently this payment stops automatically when you start claiming MA. You are allowed to have 'Keeping in Touch days' - I think it is 10 in all but you have to inform the MA people about it. You are allowed to earn money on these days. So you could quite possibly keep your hobby business going over these 10 days?
Mummy to two beautiful children, DD born Nov 06 and DS born June 09