spannerhead
They're on the back of the thrust levers. One on each lever, but only need to push one to select bump on both engines when they are running.
A4
I'm probably going to leave it until taxi now to select bump but I'm desperate to find out what causes the single bump selection. Before this happened the first time I frequently selected bump without FADEC even finishing the startup sequence and it was fine. Must be another airbus gremlin!