OK, repeating yesterday's measurements, initially with the original 530K I get 970-450 on the ADC and 4.45V - 1.90V on the DMM. With the DMM disconnected I get 1010 - 370 (very roughly, numbers jump a lot).
Replacing the 530K with a 47K I get 4.70V - 4.13V and 1010-910 on the ADC. Disconnecting the DMM has no noticeable effect.
Both these measurements imply that the lowest impedance of the receiver when working as a sensor (detecting IR reflected off a body as opposed to being stuck head on against the emitter) is around the 200K region. For some reason, I was getting much lower values on the first day of testing, along with an increased detection range (when I had the problem when disconnecting the DMM) but not any more. Oh well, still, it 's good enough and useable.