Well partial success insomuch as I'm now getting a strong reading from the sensor with the 12V feed.
I've incorporated the resistors into the circuit as per video and as per
this page too (although that page quotes 100k and 150k as opposed to 10k and 15k) resistors. I measured the output on the Z min and I'm getting 3.2v open and 4.9v closed.
The sensor registers the bed at a good distance - 10mm was achieved so I'm happy it'll detect the bed. However that's where the good news ends. It's not sending the Z stop pulse to the MKS board no matter what I try.
Firmware is all setup as it should be: pullup resistor disabled, inverted the detection (was already set to this as the previour inductive sensor was wired the same way) and all the other good auto levelling stuff mentioned in the video.
I'm a bit stumped as to where to go from here, only thing I can think of is that I've fried something on the MKS board but I'd be really surprised if I have because the only time 12V was anywhere near that was after the resistors were installed.
Any ideas?