This may well prove to be one of those "Doh!" issues....
Here's the sequence: Tx on. Plug in the battery on the F550, get all the right beeps and lights, H3-3D initialises and yes I've got a green light on the Rx. Tilt control from TX working (camera moves), flight data and camera images displaying on monitor. 7 satellites so time for lift off. Cross sticks and wait. Nothing. TX trims all at 0 and checks out in Assistant plus has been re-calibrated (again and again and...)
Plug battery back in and cross sticks and motors start. Disconnect & reconnect battery and motors won't start. Change battery and no start then unplug/plug same battery and motors start. And flies fine.
Common sense says it must be a loose connection but power is clearly getting through to the other components prior to CSC and the battery plug leads are secure to the bottom board connectors. All other plugs and connections look fine.
Anyone got any ideas or had similar issues? This is driving me