This has been on my photography wish list for some time. Finally got there. I was in position from 4.30pm. Sunset was at 6pm. I stayed on for the lovely blue hour after sunset as the bridge lights came on. I love night photography. Especially during the blue hour after a nice sunset.
I was on the south bank of The River Thames, up river side. Be aware. I was not. You need a permit in advance to use a tripod, as it is private land, on the area on the river bank path.
I got round this by balancing camera on river bank wall. More important for the after sunset longer exposures.