If you live in a downdraft area whatever you do to the chimney itself may not have the effect your after, I hope the vide I linked to is still available.
https://www.diynot.com/diy/threads/spinning-cowl.513724/#post-4269006
That's very interesting, many thanks. We do occasionally have problems with smoke seeping into the room, but this is largely due to untimely use of the kitchen extractor fan when the fire is in its early stages. Having spoken to the builder, it seems to me that because of our location there will always be a problem with smoke moving downwards after exiting the chimney, and whilst spending money on a taller pot might improve the updraft within the stove and flue, its unlikely to cure the initial problem. I could spend a lot of money trying different options and end up getting nowhere. Or my neighbour could shut his windows in the winter.