Both my RMs have one of these behind the offside bench seat back.
Iíve recently had problems with poor cranking speed when starting. This is despite having fully charged new batteries and new 50mm≤ copper cables from the switch to the batteries and between the batteries. Then I noticed that one of the battery isolating switch terminals was warm after starting. I mentioned this to at my local heritage railway as they use the same switch CAV on several of their diesel shunters. The switch develops a high resistance when the internal contacts are corroded or worn and this is apparently quite a common problem. My switch was showing a resistance of 80 ohms on the positive pole Ė far too high.
I removed the switch and the railway repaired and refurbished it for me. The resistance is miniscule now.
Starting performance is now brilliant. So, if starting is a problem, check your battery isolating switch as well as everything else.