Our project is a long way from mains electricity and wifi so the PoE ESP32 is a great fit.
I had planned on using the power from the POE for both the ESP32 and my Infrared beam sensor (it needs 12v).
I had hoped to solder onto the incoming ethernet power (under the RJ45 socket) on the board but can't identify them using a voltmeter. Looks like it should be DC+ on pin 4 or 5 and DC- on pin 7 or 8
Can you point me in the right direction please?
Not at all. What you want to do is unsafe and unrecommended.