>>>> It could be failed for a "catalytic converter not fitted where it had one fitted as standard" <<<
For the record, No it can't, this only applies to Petrol, to quote test manual
>>>> On petrol engine vehicles that qualify for a full catalyst emissions test, check the presence of the catalytic converter. Note: To ascertain whether a vehicle qualifies for a full catalyst test, use the flow charts in Section 7.3.C disregarding the result of the Basic Emissions Test. <<<<
Aidan