The brass stop tap in a flat's water supply is faulty. The brass stop tap is fixed onto the end of a fat black poly supply pipe (MDPE?). There is a poly coupler screwed into the brass stop tap with a bit of plumber's tape & this takes the main black poly supply pipe.
There is no outside stop tap for the flat.
I've never worked with this type of poly pipe. Does anyone have experience of using a freezing kit on poly pipe? It looks thick enough to handle 10 bar!