Gomog has a link in the Plus8 section that goes to the Gems manual from David Poole. It is a decent reference document and is where I found the part identification for the oxygen sensor. It's the same sensor for all four ( pre-cat and post-cat.)
My obd tool is an Autel Maxiscan and is a good one. However it is an older version that is no longer supported for upgrades to software. When connected it allows selection of vehicle make so both Rover and Landrover are available to use. The ability to display outputs whilst the engine is running helps a lot, it showed the voltage variation for my faulty sensor confirming it needing replacement. Because it was the sensor after the cat the engine was running ok as those sensors basically check the cat only so the Gems gets good feedback from the pre-cat sensor to manage the engine. The part ID on the sensor itself didn't help as I couldn't trace it. Luckily the Gems manual had usable part numbers.
Rimmer Brothers are a good source for spares.