@FlyerTalker
Our itinerary is unlikely to have been popular to begin with. We've got one way business, direct DEN to LHR then LIS, for what is, I believe, the last European SS departure ( a TA) of the season. No other reason for eastbound flights, much less one to LHR / LIS. Wouldn't make sense for SS to buy a block for that, much less one four days early. That it's one way raised the ante for a private booking. (Ah, the old days of consolidators and consolidator fares!) I couldn't touch it for the add-on SS was offering then, and the public fare is even more now.
This leads me to believe that we were the beneficiaries of a substantial general discount SS receives from United and TAP.