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12v motor causing noise in 240v system


Hi - On our small vessel we have a 12v  failry powerful motor to run the autopilot, where the power is supplied by the boat's battery bank.  On an 'independant' system we have 240v that is supplied via an inverter, but the power originates from the same battery bank as the 12v system.  The problem is that we have a laptop to run a piece of scientific equiptment, and when the laptop is plugged in to the 240v, it picks up interference whenever the 12v autopilot motor is actuated.  The laptop only has to be plugged in - the socket can be switched on or off and the interference is picked up by the laptop.  There is no interference on the laptop when not plugged in.  So some kind of noise is crossing over to the 240v wiring i guess.  

We have tried a couple things so far; adding more ferrites in various places, as well as using a voltage cleaner (a bit like a UPS) between the 240 socket and laptop but the interference still gets through.  

I've been looking at things like an IEC in-line filter that I can just plug in to the 240 and laptop cord, but these seem to only filter noise generated from mains power. I'm not an electrician so i'm a bit unclear on what the solution might be, as i'm not actually sure what kind of noise it is that's being generated.  Any advices welcome.   Thanks.