Washer Jet Nut

TD5, single-handedly managed to get the jet fitted and the nut onto the threads through the ashtray. I'll explain the trick later but am trying to tighten the nut with a 13 spanner and don't know if I'm getting anywhere. So, is the nut 13, I know I should have tried it when it was in my hand but was just to eager to get started!
Thanks in advance.....