So, the roll issue was not because of vibrations, nor was it because of interference from the ESC's!! It was in fact a defective controller:

When I connected the USB cable to investigate the PID's I noticed that the board didn't power up. It would power up and connect once the external battery was connected but then refused to let me write to the EEPROM.

I was just about to investigate further when the whole board just died.

Who would have thought that a knock off Chinese Brushless Gimbal Controller would be unreliable?!?! ;)

So I bit the bullet and found that the 8-Bit version of the official Alexmos (now BaseCam) controllers are quite a lot cheaper than they were when I bought this one, due to their new 32-Bit ARM versions being the current model.

It's also worth mentioning that BaseCam's current firmware version has an "Auto Tune" function for the gimbal PID's which I'm very exited to try out. I only hope it works on 8-Bit controllers as well!**

E.T.A. 1 week to delivery so I'll try again then.

**[edit] It appears the 8-Bit firmware does not have this option :(