Kestrel Nest at Leg o' Mutton

I'm now running on Saturday mornings rather than doing Karate due to my falling-over-a-Xmas-tree shoulder injury. Last week at LoM I found a chap photographing a Kestrel pair. The male was proud on a tall tree stump and the female in the entrance to a nesting hole in a nearby large tree (apparently these are White Poplars, the tallest species in ....London? UK? Europe?). He had been photographing them mating and commented that they had done so 3 times in a short period, which he considered more like human than animal/bird behaviour.
This week I noticed that the nest hole has an entrance at the other side of the tree too, so the female snuck in without being seen by the watchful male.