I can fit a variety of terminals to your chimney pot, (and install a pot also if needed) either to install a ventilation cap, bird guard or cowl.
If you are having trouble with puffs of smoke coming down the chimney on occasion, then you probable need an anti-downdraught cowl to be fitted on top of your chimney.
If you are no longer using a chimney, then for preventing the warm air from your room going up and out of your chimney, you should install a ventilation cap on top of the chimney and draught plate with a trickle vent a little way up the chimney to still allow a little air flow to still go up the chimney to prevent damp problems in other rooms.
If Jackdaws nest in your area, I really recommend making sure you have a bird guard fitted. These and other birds can get stuck in the chimney and die. This is not what you want decaying in your chimney.