Replacement of top clay chimney liner

Apologies if this is the wrong place for this and also if I get the wording of some stuff wrong,

The top clay liner(the one that can be seen outside) has caved in/cracked, I haven't been up on the roof as I don't like heights, but from what I can see it's only partially broken (I can still partially see parts of the chimney pot protruding from the top), I only noticed because the wire crow chimney guard was on my front lawn along with pieces of my liner, not that it's relevant but it's the open fire chimney, I lit the fire a couple of times and the smoke is exiting as normal, yet I'm going to refrain from lighting it again as I assume it wouldn't be the safe thing to do, only six months ago I had lads out cleaning both chimneys with a drill powered machine, and they checked with cameras(was looking to see if my chimneys required stainless steel liners) and everything seemed ok so they said, they install the liners and I would have thought if anything was wrong they would encourage me to allow them to install the liner, but as I said they said everything seemed ok and there were no need for liners yet.... So how would I go about it, could I just go along and get a clay pot and get a professional to replace the top chimney pot, many thanks in advance for advice...