The recent cold snap brought by northerly winds left the first frost across the south of England. The southern side of this chimney was sheltered from the wind and this is how it may have escaped a frost. It certainly had nothing to do with any heat from the chimney, no fire had been lit in it for the week before.
The other possibility is a gap in the roof’s insulation allowing more heat from the home to escape. Judging from the second frost-free patch on the right, it looks like it might be time to crawl around in the attic. Unfortunately we are battling with cluster flies up there at the moment so it is not a fun place to be. Must be a wind shadow in both cases. Definitely.