I pedal tested a Zaf 04 plate diesel the other month, standard pedal test didn't work. However, ignition to 2 (dash lights up) then pressing the pedals worked. However, it wasn't a flash code, the numbers were displayed on the odometer.
Just reading the other posts people have said they had problems or it didnt work for them. I didnt realise it was for newer engines. I did the clip test on my corsa and it worked fine. I just thought it would work on all vauxhall engines.