these are a good idea of Dive Tramps, I'd been looking at the silicone baking sheets,but couldn't decide if they were heavy duty enough to work,but these look good .
Giant Silicon Cookie Baking Mould 4 Pieces Silicone Large Novelty Cup Cake Tins | eBay they're used instead of these flashings
Dektite Premium Flashing kit for Metal Roofs 75mm - 175mm Flues RED SILICONE | eBay
for those who haven't seen them,you cut a hole in the silicone at least an inch smaller than the flue,and where it comes through the roof,stretch it over the flue and mastic and fix down the outer- this holds the flue in the centre of the large hole in the roof and stops rainwater going down the outside of the pipe,being heat resistant it also stops heat from the flue transferring to the roof