It's the one with the yellow cap on the front. Not the silver one near the centre, not sure which one the expansion is. The one on the front has a pipe coming off it, going over the top of the radiator then into the silver one in the centre, or vice versa. I could hear it circulating around this morning after it had been topped up but when it over heated I only heard it bubbling/boiling when I got out the car and saw it running out. Apologies for being so complicated lol