Hello again Ged,
Thanks for the extra info about the decanter.
Emmi is right that WF are more than likely to have made a good fit between the stopper peg and the decanter neck, most usually with a polished, tapering peg. This is, as she says, a little worrying. Together with a totally flat base it is more so, since WF would have finished a hand made piece such as this with a ground and polished concave pontil. Certainly this is the case with other items from this range of wares.
My instinct is towards this decanter not being WF and more likely Continental.
Nothing like handling an item to back one's huntches/judgement though .............so, who knows, you could prove us wrong!
Whatever it is it's a nice looking piece. Good luck with the search.