They've finally confirmed both items were held at their packing desk, not sent separately. They supplied a new packing list and parcel reference; the carrier now shows collection. I asked them to correct the order's delivery note too.

Good, an actual collection. Check both references when it arrives; the adapter name on a list won't help if a different one is in the wrapping.

The parcel arrived with both missing items. Our installer checked their references against the agreed package list, and we've closed the shortage. Thanks Pavel, asking where the adapter was packed got a straight answer at last.