Thankfully replacing the oil sensor fixed this issue - although when the part showed up I was somewhat confused. I was looking at a sensor above the oil filter and the connector only had 2 pins while the replacement had a bigger plug and 3 pins. I was, of course, staring at the oil temperature sensor. The actual oil pressure sensor is hiding behind the alternator. I figured I would have to drop that (it's a bit fiddly) but you only have to remove the EGR valve on the front of the drivers side cylinder head, drop the accessory belt and buy a long 27mm socket. Oil pressure now looks good again!