Thanks again for all the help!
Ok, so today we tried with a different lambda sensor and car warmed up:
- Voltage is now fluctuating around 0.3-0.4v with the car idling
- When we block the exhaust up, voltage goes right down to around 0.1v
- Revving takes the voltage up beyond 0.7-0.8
- Pscan still shows the o2 sensor bank as inactive
- Fuelling is at 100%
- Every now and again, the sensor state changes to active for a split second, and fuelling went down to 99.5%!
We've checked the exhaust and can't hear/feel any obvious leaks, although Dad's out there tightening everything up just to be sure.
There's also a strange, brief hissing sound just after we switch the car off. No idea!