hi nick - all I can say is that it sounds as though you have a real bargain - £4.99 is a steal for what you have there, even if yours is not c. 1810 - although from what you say it does appear it may be late Georgian.
not easy to describe in words what it is that makes something c. 1800 rather than a mid C20 copy - only experience and a lot of looking at old glass will help to understand the difference.
Colour, seeds in the glass, wear, the effects of cutting - just some of the features that help. Modern glass is, usually, too clear and bright, and wear is a big help - not just in the obvious places, but on all those parts of the glass that come into contact with other objects - if something has been around for that length of time it acquires wear (and sometimes damage) in the most unlikely of places.
Most genuinely old pieces that we find in charity shops are either missing stoppers, cloudy, or chipped (relief diamonds so often have damage to their tips) - so if you do have a genuine piece and it's in good condition then you are lucky.
I know it's expensive, but if you are into decanters, then Andy McConnell's book 'The Decanter' is a must - not just for decanters, but it's such an interesting read - Neil tells us that Andy is bringing out a new edition - but when I'm not sure. Until then a copy might cost you a lot of shekels, so might be an idea to wait for a while. Perhaps one of us should speak to the author and see if there is a time line to the project.