Yep hanging in free air next to each other flow is 16.7° and return is 15.9°. Ambient is 15.5° but they’re still in the box by the house.
I’ve now swapped them around (levered the connectors off with a flat-blade screwdriver) and will now watch my COP plummet 
Edit: I should have powered the outdoor unit off before swapping the sensors (I’m not sure how to do that though; just turn the rotary isolator? Throw the circuit breaker?) because the compressor didn’t want to start up afterwards. There were two sensor faults in the fault list in the installer menu on the VWZ AI; deleting them cleared the problem.