Agreed with the above advice.
I haven't been keeping up with this forum lately, occasionally checked in but as I don't own a RangeRover and there haven't been many relevant threads for me to join in on (LPG / engine fuelling / emissions / etc) I haven't had much to contribute.
I don't remember if Holland-and-Holland has diagnostics gear but at least if he doesn't, on Gems lambda signal wires are easily accessible on top of the petrol ECU next to the battery, only need a multimeter to check lambda readings on Gems.
If plugs / ignition are OK and lambda readings are OK, other things that can cause high CO / HC readings are individual cylinder mixtures. Lambda readings represent close to average mixture readings over 4 cylinders but if some cylinders are running richer/leaner than others then high HC and/or CO can result. Things that can cause that are worn injectors and/or valve gear, the former effecting petrol flow, the latter effecting airflow. I have seen many P38s run more efficiently when running on LPG than when running on petrol (e.g. lambda readings correct in both cases but the engine draw more manifold vacuum when running on LPG, which implies the engine needs less air when running on LPG, which points to an increase in efficiency at least for idle conditions) - I put this down to petrol injectors being old and worn, LPG injectors (on a new install) being new and equally balanced in terms of flow.
Different but closely related subject - On another forum (for a vehicle I've LPG converted hundreds of examples of) I've advised many owners that they don't need cats to be intact if they are converted to LPG. These vehicles are prone to failed cats, they have 4 cats (2 at each side of the V6), front cats fail then the debris clogs the entrance to the honeycomb structure of rear cats, preventing exhaust gasses escaping, causing back pressure, back pressure blows some of the (ceramic) cat dust back into engine cylinders where it acts as a grinding paste and ruins the engine. Many owners have had the front 2 cats, rear 2 cats or all 4 cats decored (4 cats only in case of LPG converted), mostly on my advice. Recently an owner who had decored all 4 cats took his vehicle for MOT but didn't tell the tester it was running on LPG, so the MOT tester failed it on emissions. This was a bit of a sticky moment for me as it could have ended with me being slated on this forum but I had faith it would work out OK in the end.. which it did. The vehicle owner then took the vehicle for MOT at another testing station, told the new tester it was running on LPG, it easily passed the emissions test. It could not have passed the MOT running on petrol with all 4 cats decored.