i don't understand where they are getting this from its only a month ?
What's your accounting date?
Just tell HMRC that your accounting date is 31 August, you've presumably filed the 2011 tax return by the 31 August 2012 and HMRC have their dates wrong and the penalty isn't correct. It's all automated so they spam out these penalties quite often by mistake.
Edit: I also assume you filed 2 tax returns for the period ending 31 August 2011 or you were dormant for part of that period?
A tax return can cover a maximum of 12 months, if you incorporated on 10 August 2010 and your first accounting period was until 31 August 2011 then that's longer than 12 months and 2 tax returns are required.
Unless you incorporated and didn't begin to trade immediately.
Did you prepare the tax return yourself or did you pay someone to do it for you?
It sounds like you should have submitted 2 tax returns. HMRC have got the dates wrong but that just means the penalty should have been issued sooner if you only filed 1 return.