So, it seems that YTB were incubating eggs right under our camera lens (or, literally, above)! Thanks to TomT for noticing and persevering, and Michael M for poling and discovering two eggs that are currently being incubated. We've noticed a family of three YTBs regularly visiting the area (although on one sighting there were two youngsters - perhaps one was from 2024?). The female would poke her head out of the hollow - just as she did last year, so I dismissed the possibility of breeding - but I have to eat my words! And perhaps they were successful last year, unbeknownst to us. I have included a photo of the female from January 7.