An interesting turn of events: both 4k monitors — 2 shipments from 2 senders — were grouped by Parcel Force into the same delivery (the boxes were literally taped together), which was signed for by the concierge (who obviously don't have the authority to refuse any shipments).
Which means the seller in Germany now has to be the one to arrange for a return collection.
Also, wow. 1 4k monitor is already quite a lot of screen space, and I have it scaled down one level for my not-amazing eyesight.