This car is failing its emissions test with the following results at fast idle
First test:
O2 1.22%
CO2 12.0%
HC 439 ppm
CO 1.26%
Lambda 1.005

Second test (two mins later)
O2 0.84%
CO2 13.3%
HC 329ppm
CO 1.19%
Lambda 0.995

This is the 8 valve single point injection engine so I'm not expecting brilliant results, particularly on HC's.
No fault codes in memory.
Live data shows lambda sensor switches 1-2 Hz at idle, coolant sensor is recording correctly.
No detectable exhaust leaks, particularly upstream of lambda sensor.
Slight misfire (well I thought so anyway) so I fitted new leads, plugs had been changed during a service last week.
I advised customer that the Catalyst has failed.
Customer calls and tells me a new catalyst is £500 genuine, £250 non-gen, how sure am I it is the catalyst.
Now I am concerned that my diagnosis was wrong but I can't see any other reason as the lambda is very close to 1.00. My first thought was airleak upstream of lambda sensor causing the management to richen up but I discounted that early on.
My question is do the gas readings point to a faulty catalyst or me being an idiot?
Another thought, could it be wrong plugs giving poor combustion (BKR6EZ fitted from memory, which according to my NGK book is correct)?
Shaun