Ooo I am sorry to hijack...
I didn't claim FB A last year (DS was born 2/5/11) because I hadn't done 09/10 tax and thought I couldn't claim it if I hadnt done previous years tax (i dunno if this is true or just my warped thinking)...
So can I claim it when I do 10/11 tax return? from 2/5 to 30/6?

Also, I havent claimed A+B so far this year cos again... fear over not having done 09/10 tax and 10/11 tax. Will I still get all the money eventually though (and backdated) once i've sorted my tax and made my claims?
yes you can claim for the period 02.05.11-30.06.11. you need to claim and have lodged your income tax returns by 30.06.13 for that period
for the current year, you can claim at any time - but if you don't lodge this years tax returns within the normal processing time, you may end up having payments suspended and debts raised until your taxes are done. if you're doing ok without it, you can always claim after the end of the financial year (for the current 11/12 year you can claim up until 30.06.2014)