As far as I know, based on my friend's applications, one would have to wait at least 12 months after ILR to qualify for Naturalisation. You would likely loose your application fee if you submit too early.

Also, the link pointed out by Darren states that firmly too.

However it also states, "There is discretion to allow applications from people who do not meet this requirement. For details of how we apply discretion, you should read the page on discretion to regard immigration time restrictions during the last 12 months."

I do not know what the special cases are - likely marriage to a British.