Last MoT figures were CO 0.02%, HC 24ppm
What he's talking about is CO2 which is inherently lower on a diesel engine than a petrol engine which is why diesels became popular in the first place. The CO2 when running on LPG is 10-20% lower than when running on petrol but still higher than an equivalent diesel. However, although CO2 is a greenhouse gas it is the other things that are of most concern these days. The particulate emissions on a diesel P38 will be very high (as it is too old to have a DPF), considerably lower on a petrol P38 and virtually non-existent on a petrol engine running on LPG. It's also a far less complex hydrocarbon than petrol without any of the additives petrol needs to have so the NOx and other carcinogenic emissions are virtually nil too.