Sorry jay jay for the eggsucking lesson.
I am not an authority but have not personally encountered asbestos used in timber fire doors except as a later "upgrade" and it can be very difficult to determine whether the door lining is something harmless like supalux or whether it has asbestos content. Only analysis will tell. In some circumstances I have seen panelled doors with the panels only infilled with plasterboard or asbestos sheets, then the lot covered by a piece of hardboard. This was a common method of upgrade and modernisation (in accordance with BS459 I think) in the early sixties.
I have often seen asbestos and steel sandwich doors usually as compartment doors in factories.
Timber flush doors are usually a solid core slab or a mass of wood strips glued together with decorative faccing material, and lipped, or cheaper doors had a chipboard or flaxboard core in strips glued together with an internal framing, or even wood particle cores that may be made from a single piece of board.
Personally have never seen asbestos introduced into a timber door by the manufacturer.
Not much help again. Sorry.
