I was sort of lucky enough to see another one leave later the same day. It headed straight out of the garage and up towards the roof of one of our neighbours houses. It misjudged the landing, head butted the top of the wall and then fluttered and fell on to the parapet a floor below. I think that it was okay, but it was a very advanced manoeuvre for its first ever flight.
The final one really didn't want to leave and spent the whole of yesterday standing sadly on the ledge next to the nest. Very poor photo below.
It eventually left when I accidentally disturbed it. I thought that it had already gone and so I walked in to the garage and saw it sitting on the floor. It squeaked really loudly and angrily at me, and then it was up and gone.
It's very quiet without them, but at least I can now get on with insulating and tidying the garage.
I spent most of today up a ladder trying to make the edges air tight, and I just need to put two layers of insulation between the joists and then it can be plasterboarded.
Not an exciting photo of the inside of the garage, but that's what I've been doing, so I had no choice.