Thanks for the replies - yeah, the car is taken for a good long run before the test is conducted, but no, I haven't tried any cleaners..
I totally understand the safest option being OEM for the test - just the cost of purchasing them is a little eye-watering at the moment, as the original ones were sold.
I'll have a look at... ISR, you say?
I'm not sure a straight pipe is going to suit sadly, or the interchangeable Y-Pipe sounds interesting.