A few more things to now check off the list:
Fuses checked - all good, tight connections.
Fuel filter - totally clear.
Spark plugs - torqued and gapped correctly. ( I was really holding out hope for this one, but no...)
There is one electical connector by the carbs coming down from the frame that doesn't connect to anything. It is female, 2 prong with one purple and one black wire. I believe it is for a 12v accessory outlet though - can anyone confirm this for me?