A pair of magpies, which started nesting nearby, clearly realized there's something going on (by the blackbirds flying in & out from the nest) and investigated the area, but luckily they have haven't discovered the blackbird nest yet.
In very dry gravel bed along house wall (and partially protected from rain by roof overhang).
Pretty sure it arrived there on its own rather than being planted.
Leaning towards T. campestre: www.blumeninschwaben.de/Zweikeimblaettrige/Schmetterlingsbluetler/trifolium_gelb.htm
Near hedge row. Fairly large colony with mound (hill). Looked different from www.inaturalist.org/observations/3273039
Quite large - Aglia tau, or Saturnia pavonia?
There were quite a few of these, which unfortunately would never settle but continuously fly up and down. In the end I decided to just point and shoot without focussing, hence the poor photos. Better these than nothing, though...