1. You may apply for extension anytime you wish. Any leave granted, if it is not in line with the specified immigration rules, can be corrected.
2. As already suggested, it is general consensus that it is ideal to apply for extension in the last 28 days of current leave so there is no shortfall in becoming eligible for settlement. This however is not relevant in your case because you just need a few months extension to be able to apply for settlement.
So, essentially it is your choice when you should apply for extension.
If you wish to save money and applying by post is not something that worries you then my suggestion to you would be that you apply for extension just a couple of days before 05-05-13 (keep proof of posting safe). If your postal applications for extension is not decided by 01-09-13 you should then vary your extension application to settlement application. See
Varying an application.
I cannot speculate of the processing times of extension application and cannot say for sure if your application will or will not be decided by 01-09-13, should you wish to take this route, but thought it wise to pen down the option for your information.
See also
Child born in the UK.