Hi
Yes you are entitled to 52 weeks Mat Leave (26 Ordinary & 26 Additional). As a previous poster said, you are still employed even when on mat leave.
In Sept you will be between 17-20 weeks pregnant. So, (depending on whether you return at beginning of Sept or end) you will be able to go back for a minimum of 9-12 weeks, but don't forget you will need to use all of your Annual Leave entitlement before you go, that you have accrued, as you won't be returning to work again before April 2008.
The only consideration is that your SMP is calculated "in the 8 weeks leading up to and including the 15th week before your expected date of delivery". Basically, they will average the pay you receive between when you are 17 weeks pregnant and 25 weeks pregnant, and if that is over £112.75, you get 90% of average for 6 weeks then £112.75 for 33 weeks. If it is under £112.75 but over £87 you will get 90% for 39 weeks. If under £87, you qualify for Maternity Allowance, but not SMP.
So, you need to make sure that you go back to work at 17 weeks pregnant if you aren't earning masses, to maximise your average over the 8 week calculation period (So, for you, the beginning of September).