Good suggestion. Fortunately there is another field BuildingID identifying the building to which each facilities belong so the delete is simple, just
DELETE FROM facilitylink WHERE BuildingID = $_POST.BuildingID
followed by multiple inserts of
INSERT INTO facilitylink (BuildingID, FacilityID) VALUES ($_POST.BuildingID, $value)
it was the $value bit i missed, kicking myself now, should have spotted that before asking, sorry
Got it fully operational now, thanks for your help