Radiator cap/ expansion chamber cap

Which is which??

Whilst the rebuild was taking place, the caps got removed and now I am not sure that we have them the right way round. I have the flat one on the radiator and the pressure release one on the expansion chamber. Is this the right way????

Flat one on rad, yes - spring cap goes on rail at top of engine - though if you've changed to a pressurised expansion vessel it should probably go on that.

Spring Caps should be able to go on both, but the cap under the passenger seat has to be a sprung one.