The new substation (
TF0820 : New substation installation) carries this warning notice about SF6, whereas the old one carried a warning notice about PCBs (
TF0820 : Substation warning notice)
SF6, Sulphur Hexaflouride, is not particularly dangerous. It is used as an insulating gas in high voltage switchgear. But in the event of a fire the combusion products of SF6 can be very nasty indeed, much like the substances used as gas weapons in the first world war.
I don't think there is any cause for concern here. The volume will be small, and in the event of a fire I would have thought the other combustion products would be just as worrying.