Rob White

Thanks all. I have this problem. Canon's response is that the 550D is less feature rich than the 7D and thus the option is simply not available.
I've got the flash power dialed down to it's lowest setting but I'm still having major issues with "Busy" messages from the camera.
If I cant confidently hack the firmware to enable the feature, I may resort to switching the flash off toward the end of each dive and start shooting natural light subjects instead!

I'd just like to add my findings.
I was lucky enough to have the first Nauticam 550D housing off the production line.
I dived with it 4 times a day on a weeks Red-Sea live-aboard trip, on a 2 day trip to the North of England, a few times in the local aquarium and numerous times off the beach of the South coast of England.
The "zoom in" control rod on the housing didnt quite reach the camera button inside but that issue was resolved really quickly, both temporarily so I could use the unit on holiday and then permanently, once I was back in the UK. There was no ingress of water to the unit despite this teething issue.

I've had absolutley no problems with any leaks, nor any issues with loose screws.

I am over the moon with my Nauticam housing and I'd recommend them to anyone.

Hope that helps