Snowlet - I did it all through the post. Filled out the form on the computer at 26 weeks, printed it out, signed it, and then posted it.
I've yet to receive a penny, but I have had the letter telling me what I am entitled to at least.. I then had to fill in another form to say when I wanted it to start from despite writing this down in the "comments" box on the first form 
That was ten days ago and I've heard nothing else yet. So far they owe me 3 weeks worth backdated and counting..
Rootvegetables - this is true, you do need to have been working a certain amount of time. Have you done any paid work at all in the last 66 weeks before your due date? As if you've done 26 weeks somewhere in all that, you will qualify in some way. You can backdate your self employment registration to the date you got your certificate as that's when you would have been legally available for work.
lucyfluff - It is ridiculously confusing how they word it. Honestly, I was sure I wasn't going to get a penny from looking at it several times, and I wondered how I managed to before. Then I spotted some small section somewhere and remembered, but they do not make it clear or obvious AT ALL. I'm sure that's deliberate.
For instance, there's this taken from here: www.gov.uk/maternity-allowance/eligibility
"You might get Maternity Allowance if:
- you’re employed, but you can’t get Statutory Maternity Pay
- you’re self-employed and paying Class 2 National Insurance contributions
- you’re self-employed and have a Small Earnings Exception certificate
- you’ve recently stopped working
Also, you must have:
- been employed or self-employed for at least 26 weeks in the 66 weeks before the week your baby is due
- been earning at least £30 a week over any 13 week period"
That, and a similar sort of document elsewhere, strongly implies, no, it actually STATES or at least seems to (there's no "or" mentioned) that being self employed means you need to be earning over £30/week. There is NO mention of anything else. In fact if you click on their eligibility calculator, it says I would get nothing. Yep.
Yet if you look here, on a link hidden away, it's another story: www.nidirect.gov.uk/maternity-allowance-notes.pdf?rev=4
"If you are self-employed you are required by law
to register as self-employed with HM Revenue &
Customs. If you do not register, or register late, you
may lose some or all of your entitlement to Maternity
Allowance.
If you
- are registered as self-employed, and
- have paid Class 2 National Insurance contributions,
and
- do not hold a Small Earnings Exception certificate
you will be treated as having enough weekly
earnings to result in the standard rate of MA for
any week covered by that"
And this is HIDDEN in a PDF under a link that you only click on if you go on to apply, not in any of the so-called "clear" headline documents. You only find it if you dig, and frankly who is going to dig for it if you didn't know it was there? You'd believe the first thing you read of course especially after running the calculator, and not bother trying to apply.
Nevertheless, it is true, there IS no minimum or they wouldn't have paid me a penny for my last pregnancy nor authorise it for this one either.
Gits for being so misleading, ambiguous and hiding stuff away, honestly.
And let's not get started on the whole Small Earnings Exception Certificate thing as in the OP.. another bunch of misleading and potentially very costly crap too! 
Anyway, I hope all that helps somebody!! 