Cannon Broken lens attachment - PLASTIC!?

I have a Cannon 400D and my better half a Cannon 450D with the standard lens kits.

I dropped the 450D recently onto a couch with the EF 50-275mm lens attached and it has split the PLASTIC part of the lens where it attaches to the camera body, the one held by four screws to the lens itself.

On my 400D the lens is the EF 75-300mm and of course the same part is NOT PLASTIC so the above probably would not have happened!!

I was after a part number for the above and/or how to go about a fix for same.

I live close to Brisbane Qld Australia also ;)