So I have finalised this. The new grey connector sensors are indeed after-cat sensors, as they showed 0v on the OBD readout. I put the old ones (orange connectors) back in one at a time and they instantly started showing values (between 0.1v and 0.9v) and then the fuel system readout on OBD went to "closed loop - using o2".
So I am happy, the O2 sensors are working again, and I am back on closed loop operation. However, be careful when you buy o2 sensors, as the website I bought them off clearly mislabelled them as pre-cat o2 sensors and while they fit, they dont work.

