It occurred to me that I had another smaller gearmotor by the same manufacturer, link here and motor curves.
I tried swapping it for the gearmotor which seems to be giving me all the trouble and to my amazement, it will run alongside motors A & B without interfering with them, perhaps a slight pulse from motors A & B when I turn the gearmotor switch on, but that's it.
Now, I'm not sure if it's strong enough for the same purpose, but does it suggest that the interference/noise is all coming from the other gearmotor. If so, what would be the best steps to try and stop it?
Thanks
Michael