CurrentCulture incorrectly defaulting to en-US in

I have just migrated around 100 sites from IIS 6 on Windows Sever 2003 to IIS 7 on Windows 2008. I’ve just noticed that various pieces of code that use things like DateTime.Parse have started kicking up errors “String was not recognized as a valid DateTime”. I’ve tracked this down to the fact that the CurrentCulture of the sites is defaulting to ‘en-US’ and so my UK users are inputting dates in an unexpected format.