Hi m_c, thanks for all the help you've given. Appreciate it.

The sensors are these types:

DC 10-30V 4mm Inductive Green Proximity Sensor Switch 3-wire PNP SN04-P2 | eBay

Sadly no data sheet and the specs are basic at best. I'd say that they're active outputs as I had them test wired this evening and when you tie all the outputs together they do strange things. There's a little led on each to show active, when wired in this manner you have to trip a couple of them in order to get things going. Wired individually they work fine.

Could I place a diode after each output or is this not a case of current flowing back to one another?