Hi if your chimney pot is 190mm is diameter the Redbank cowl is a good option although, I would recommend https://www.roofingsuperstore.co.uk/product/colt-cowls-econocowl-antidowndraught-cowl-150mm-to-250mm.html this cowl as it adjustable for most chimney pots, cheaper and easier to fit.
Thanks S&S Roofing and Construction