Thanks to Jude for spotting this page on the London Transport website:
“Any Travelcard or LT Card is valid for travel across the entire London bus network.
For example, with a 7 Day Travelcard for Zone 3 you may travel within Zone 3 by Tube, Tramlink, DLR and National Rail, but you may travel across the entire London bus network.”
… meaning that my zone 1-2 travelcard will work for all the London bus routes!
And I’ve been paying everytime I go outside Zone 2 for *years*. grrr.