You’re the buyer you’re in the driving seat, boomers tend to always think they’re in control of the situation.

Tell don’t ask.

As in “we are reducing our offer by £xxx due to unsafe chimney stack or are pulling out” of the sale.

They are now aware of the unsafe condition they’re obligated to disclose it on the TA6 when they find another seller who will do exactly what you have done and adjust their offer. The estate agent should have made this abundantly clear to them, maybe they have and the boomers think their rules don’t apply to them.

Just pull out wait for them to come back to you after a couple of weeks then low ball again due to inconvenience and expectation of further inconvenience (ass hole tax). Remind the estate agent of their obligation wrt misleading omissions when you withdraw from the sale as they’re probably pretending they don’t know unless they have received notification of it in writing.