HMRC agreed the 2008/9 tax was finalised and no further enquiries or claims will be made into them.
That seems unlikely. Have another look at the letters they sent you at the close of the investigation. They may say that the enquiry is "closed" but would not say that no further enquiries etc would ever be made. Indeed, it is more likely - if it was a full investigation - that they will have asked you to sign a certificate of full disclosure (CFD), which will carry the warning that if anything further ever did come to light, the consequences for you would be serious. The CFD covers all aspects of the return, not just the area enquired into.
In any event, HMRC have until 5 April 2013 to raise a discovery assessment into 2008/09, or until 5 April 2015 if they can make a case for careless behaviour. Their right for HMRC to raise a discovery assessment does not disappear just because an enquiry has been completed already. The main defence against a discovery assessment is that your tax return gave full information about the area enquired into, or that the assessment is out of time. If HMRC are on the ball, they will raise a discovery assessment by 5 April 2013. If not, you could point out that they are out of time for raising an assessment. But this would be an aggressive and risky strategy if you have signed a CFD, as there is then a higher risk of criminal prosecution being taken against you.