richgk wrote:
The house is 1975 build is there a method to see whether it is already
installed?

No, my mother recently took advantage of the over 70's offer of free cavity
wall insulation - my late father desinged and managed the build of our house
in 1971 - when the installation team arrived they drilled into the cavilty
to find that the cavity was already filled with polysterene sheets.

Jon