Thanks for that Lawrence

As the clay flue has a 200mm diameter and the spiggot thats butting up
against it is 140mm , I'll need to taper whatever "sealant" I use at
this point so that the "tars" can run back down the chimney into the
fire. Do you know if it is possible to build this ammont of silicone
at this join? Also as I'll be sealing after the spiggot has been
fitted (which means I only have access throught the 140mm hole in the
spiggot) the "sealant" will have to be applied by hand (no roon for
gun). Being only familiar with the normal silicone products, this
would appear to be a messy job would it not?