Thank you both. I understand the issue with a rubber washer on top potentially looking unsightly, but I do agree that the washer being on top saves fiddling around with silicone or, as you suggest, Plumbers Mait. As noted in my first post, when I bought my last set of taps I found the instructions to be indicating that the washer should be put underneath the sink; hence my question being prompted by subsequent problems I've had with the tap "swinging". Probably should have gone with my instinct and put it on top, but you tend to take the view that manufacturers know best about how to install their products.
Moving on from that, another question. I've bought taps from a certain well known DIY warehouse on several occasions, and they've never lasted more than 12-18 months without one of the valves, usually the cold tap, starting to feel rough in operation or ceasing to operate altogether. The triumph of hope over experience has made me go back to them again and again. Seems to me that I need to look somewhere else to buy bog standard basin taps, so any recommendations gratefully received.