I've just had this car back in to remove the front bumper and inspect the loom. With the bumper off, I found that it did have secondary air injection as the air pump is below the n/s headlamp. I followed the air pipe across the front of the engine and found a vacuum operated valve. The actuator that controls this valve had rusted and split. Horrid to get to as its mounted on a bracket attached the front of the engine just above the sump. Anyway, £10 actuator has now cured the fault. Happy days!