They acknowledge the full omitted kit and its adapter reference, but have only proposed attaching it to another order, which we have not placed and do not plan to place.

Then ask for a separate-supply proposal. Another purchase isn't an agreed prerequisite for receiving this kit.

Parts confirmed stock and separate shipment at the seller's cost, which we accepted, and the matching packing list is now with purchasing; no carrier collection evidence yet, so I have not told maintenance it is travelling.

Did that arrangement result in a received kit, Thomas? There has been enough time for the status to change, but the earlier packing list does not tell us whether collection or delivery actually followed.

Yes, carrier collection and delivery followed, and stores has now checked the complete kit against our accepted list, including the additional adapter's reference; the shortage is closed on actual receipt, with the spare recorded as stock rather than an installed replacement.

Good to hear it reached the cupboard, not merely another shipping promise.