Does the negative terminal of your 12 volt battery need to be connected to the GND pin of the arduino? If it needs to be connected, then it just looks like your existing fritzing diagram doesn't have it connected.
4) My understanding of a bridge rectifier is to switch ac to dc. So would it not in fact stop the system from reversing polarity to the actuator when required?
5) Can anyone explain why the 12v output from the relays would switch to a negative value, less than required (-0.7 to -8v). Is this because of the long leads between the relays and the actuator? (2.5meters).
Not sure what the intention of that diagram is, but it has nothing whatsoever to do with what I was explaining about using a bridge rectifier as a convenient form of voltage limiting for the relay switching.
It is in fact, nonsensical!