We have suffered from a small amount of water creeping in the chimney flue. It ran down the outside of the flue pipe, suggesting there was some sealing issue with the rain collar supplied with the flue.
What we weren’t aware of when we fitted the flue was that the collar needs high temperature silicone to ensure a totally watertight seal. So it was up onto the sedum roof and off with the collar and out with the silicon gun.
As we had a whole tube of sealant and did not want to go up again, we put the high temperature silicon on the obvious places and the less obvious places to make sure we did not have to get the ladders out and up on the roof again in a hurry.