I came across Grade D detection in the communal areas of a purpose built block of flats, it was not interlinked to the flats, would this be described as a part 1 or a part 6 system as it was communal areas?

It's a part 6 system that serves no purpose other to draw people into a smoke filled stair, usually these are either withdrawn or replaced with a compliant full evacuate system (i.e. sounders and detectors in flat lobbies) depending on the results of the FRA.

You can't build a compliant Part 1 system using Grade D equipment so it can't be categorised as such.

Thankyou Anthony.