Stephen, you wrote “David presumably your switch, being 4 way, is a rather more sophisticated circuit arrangement?” ..No; it’s extremely simple. (See that web page http://tinyurl.com/ybtlzgw8 )
It has positions 1 and 2, with terminal 1 connected to the starter batt positive and terminal 2 connected to the “house” batts positive.
There is a 3rd terminal connected to the starter system positive.
In position 1, the normal starter batt starts the engine ..as it usually does. In position 2 the “house” batts start the motor. In position “Both” the terminals 1 & 2 are connected together and BOTH start the motor (if necessary). When the motor starts, because its relay connects both sets of batteries, that’s exactly the same as the “Both” position on this selector switch. That’s it. Simple.
The negative cable isn’t attached to this switch ..the neg just runs as normal from both batts to the engine casing / starter motor casing and the rest of the negative circuitry.