I have used the Sidcup line for twenty five years and do not recall regular timetabled use of the Lee Spur before even during engineering works. My only experience of it before this weekend was occasional unplanned diversions when lines through Hither Green were blocked. I certainly have no recollection of there ever being timetabled services using the Orpington, Sidcup and Greenwich lines in this way. So yes Ian, you may well be right. Having used a couple of times over the weekend, I was surprised how many people were using the route despite greatly elongated journey times from Sidcup and Orpington lines to central London. Rather enterprising of Southeastern.
You may also be correct in suggesting that use of the Thameslink platforms at London Bridge for Charing Cross services is a first, at least for timetabled services.
David