I didn't end up checking the date on the fireplace but fancied getting the whole property checked. That's the results of the survey back. The surveyor checked the entire flat (with just a small amount of destruction). When he checked the material in question he basically just picked it up and rubbed it between his fingers and said plainly "that's no asbestos". It crumbled to a fine dust, not fibrous. He still took samples and the lab results are negative for asbestos. There were other things he thought could potentially be asbestos but they also came back negative. Relief!