thanks Bernard

Rita - my apologies, I obviously wasn't looking carefully enough, and hadn't seen the concave pontil mark. Beyond the rather bog standard pointers about decanters i.e. neck ring patterns - type of panel cuts, mushroom or spire stoppers, and the matching Nos. etc., I know very little about them specifically, so maybe someone else will be able to help further. I understand that there is a tendency for genuine C18 and very early C19 examples to show a slightly greyer colour of glass.
As to value, I think most of us would agee that this is too subjective - very much dependent on how much someone wants something - so really unable to help you there. Places like charity shops and boot sales are sometimes awash with these things - although they are often chipped and stained internally, thus reducing their appeal. Assuming yours is the original stopper, then this is a plus - as so often stoppers have been lost or damaged and the replacement looks wrong (and this is where the Nos. come in - as if they don't match, then they didn't start out in life together). Just fill yours with sherry, and enjoy its beauty (and of course drink the sherry)