Expiration of a reserve matters approval pursuant to an oultine permission - Public forum - Planning Advisory Service (PAS)

I am looking at a site which is being developed in phases. Outline permission was granted in 2000 and since a number of reserve matter applications have come forward for various plots. The outline planning permission has been lawfully implemented Iam told, but I just wondered if it were possible for a reserve matters approval to lapse/expire if not implemented? Would I be correct in thinking that the reserve matters approval is not explicitly a grant of planning permission in itself so therefore cannot expire if the outline permission to which relates has been lawfully implemented?