Thanks for your help.
I'm using the standard arrangement of a power module to provide power to the PixHawk and the BEC in my (quad) ESC powering the servo rail via single attachment point.
As you can see from my first trace, the servo (BEC) line stays stable the whole time. The main bus seems to start then immediately shut down a couple of times.
This may be due to an excessive load at switch on or just some instablity somewhere in the switching circuit.
I've seen it happen on a chinese clone as well with a different receiver which is what makes me think it may be an issue with the reference design.
The downside of my workaround is that I lose the redundant supply feature of the PixHawk. However, I don't think this is much of a loss since all of my power comes from the main LiPo.
You're probably right about the noise resistance of the receiver, though servos are notoriously noisy beasts so I'll have to wait and see.
To give a little context my background is in both electronics and software, though mainly digital rather than analogue systems.