No output from X via Multibridge

So I've taken the plunge and upgrade the Mac to take Mavericks - SSD with the SFOTT key used to install Mavs, ATI5770 card - all lovely. FCP7 fine, Color fine, Resolve fine, FCPX runs - but no video output from the Multibridge. It's there in the output tab in Prefs, but no image.

Any clues as to why that might be?

BTW I can really recommend upgrading if you're looking for a new lease of life for an old machine. Cost so far has been around £300 (new GPU card, SSD and adaptor) and it's fine - nice and snappy for a 7/8 year old machine, and a lot cheaper than a nMP!! It was a ludicrously easy process too.

Anyone had any joy with this problem? I still can't get it to work. Latest BM drivers on latest osx with latest version of FCPX...