They may not have failed at the same time - I've had the car for a couple of years and the warning light was on before I got it for at least another year or so. To test them I'm guessing I'll need to dig out my voltmeter then get the car up on stands and trace the wiring?

The fault codes you mention relate to O2 sensor circuits for both heater and supply. Have you checked to make sure both sensors have a 12v supply? If not check all fuses and wiring. IIRC both white wires on the sensors are live supply, one for the heater and one for the sensor. That leaves a grey wire and a black wire. One is earth and the other is the signal wire, but I can't remember which is which. You'll have to test them and find out. I wouldn't be too quick to change the sensors. Both of them failing for the same reason at the same time is probably a bit too much of a coincidence. A voltage supply fault is much more likely.

Fuses are all fine and the supply to the connectors reads around 12V. The wires to the post cat O2 sensors look ok from what I can see but actually getting access to the sensors to get them out and look at them seems like it's going to be a massive ball ache as getting it up on axle stands doesn't give me enough space to manoeuvre tools, hands and elbows to where they need to be.

Anyone here got any experience replacing post cat O2 sensors and got any advice on how they did it?

The fault codes you mention relate to O2 sensor circuits for both heater and supply. Have you checked to make sure both sensors have a 12v supply? If not check all fuses and wiring. IIRC both white wires on the sensors are live supply, one for the heater and one for the sensor. That leaves a grey wire and a black wire. One is earth and the other is the signal wire, but I can't remember which is which. You'll have to test them and find out. I wouldn't be too quick to change the sensors. Both of them failing for the same reason at the same time is probably a bit too much of a coincidence. A voltage supply fault is much more likely.