Discovery 2 V8 2002 with LPG. Disaster purchase. Failed MOT high CO reading

Thanks for that. Gonna do the head first. Also, I don't think the secondary air is working. Will take the pump off to test.

Secondary air pump is knackered. Filter has disintegrated and fan will not turn. Well rusted up. Does the fan turn off after the cat is warm enough or does is continue to pump air through to lower emissions?
I've tried to dissemble the pump. The fan blade will not come off. I guess too much rust.
I've seen a few pumps on 'the bay' but although similar they seem to have a slightly different build on the filter cover and are for range rovers. They look to have a further pipe attachment there.

Head gaskets replaced and head skimmed. On disassembly found that previous owner had got the lpg feed pipes to the front and second, bank 1 cylinders swapped. I was hoping that this would be the cause of high CO readings (2.6) but it would appear not. Also, I have replaced the secondary air pump £18 off the bay and repaired vacuum pipes.
Still getting fault codes for the 2 pre cat lambdas. I want to drop the section of exhaust that houses them to replace. However, I am having trouble getting the electrical connections for the post cat sensors unclipped. On the drivers side, the loom side of the cable is extremely tight and it is nigh on impossible for me to get my hands in to unclip. I managed to get it off the tab but even getting that back on will be a struggle. Anyone any suggestions?

Don't know if it's the correct way. Managed to get the passenger one off by leaving it in place on its tab, squeezing the clip lift and easing it apart with a screwdriver. They stick together pretty tight. Access to the driver side is much more difficult but will attempt this method. Have dropped the exhaust to one side to be able to get my hands in better. To remove the exhaust completely, I assume I'll need to remove the bar that goes underneath the cats from one side of the subframe to the other?

Managed to get the last electrical connection off using the above method but it was extremely awkward even with the exhaust dropped. The bar under the exhaust had to come off. I found 2 of the bolts had been sheared off on one side and another had been rounded off on the other. Fortunately, a while back I'd bought a set of easy outs for rounded off nuts which worked a treat.
So I have the exhaust off. Waiting now for the delivery of the lambda sensor removal tool to get them out. I've soaked in plusgas but think it likely I'll have to get some heat on them to remove for testing.