Ok, so back into this now. Had to revert to the 4.0.0 flex SDK. This compiled straight away.
To download this, left click-> properties on the project in flash builder.
Choose flex compiler, if 4.0 is not available, click the configure SDK's link.
In this page you can add additional SDK versions and there is also a link to download other SDK versions.
I had to download 4.0.0, unzip to the SDK folder and add this, apply changes
The 4.0 should now be available in the 'use specific SDK' dropdown on the flex compiler page.
Regards
Andrew