I have bought all of these in the past. The silicone stoppers from Axminster do require a taper on the foot of the bottle stopper, and some glue, but if there's a slight recess right at the top of the taper, they do sit very neatly, so I don't agree with Paul that they are no good.
The cheap ones from ebay are easy to work with, but the quality is mixed, you get some where the thread isn't straight and the result is a little untidy. The Axminster metal stoppers are good and give a tidy finish, but are a bugger to work with.