I installed sql server 2008 R2 Express successfully, then realised I need to get Full Text Search. So I downloaded the "Advanced Services Installation Package", but when I run it there is no option in the "Feature Selection" part for Full Text Search. Please don't tell me I hav to uninstall and reinstall?
No, unfortunately you'll have to install a new instance with the Advanced Services installation. In R2 you can only add features such as Full Text Search if you initially installed with the advanced services installation.
You could backup your database though and restore it to the new installation w/ Full Text Search. Or I think also detach/attach.