Hob Grate open fire into 270 year old brick flue. 

No it's a different one.

That's it.

Chimneys pots are bedded in a lot mortar (it’s called the chimney cap/flaunching). If the pot is loose enough for a chimney brush to move it the chimney will akready have a lot of problems with water ingress and the homeowner will know something is wrong.