The emergency switch on the middle carb is not easy to see or get to from outside the boat but putting it in the 'Emergency' start position worked for me. You must remember to return it to the Normal Position after the engine is warm (say 40 - 60 seconds) as I am informed that the oil pump is not operating on the middle cylinder with it in the emergency position.
Her are the two positions from my Engine. You need to spot the little red switch. The pic on the left is the switch in the normal position and the pic on the right is with it in the Emergency start position.
You just flick it with your finger or in my case a short length of 6 mm dowel