Hi Phil,
Sticking my head above the ramparts and waiting to get it shot off...
It is difficult to invest in tools in the current climate I know, I couldn't afford to buy a smoke machine, so I made one and it has proved very useful too. But what about the time honoured method of getting someone to stuff a rag up the exhaust and listening carefully. I suspect you have an exhaust leak before the first lambda sensor. What lambda value did you get as a high CO and a high lambda is often a giveaway.
PM me if you would like some details on my smoke machine.
HTH Shaun