Firstly. welcome to the Forum !
Secondly, the bee in your photo is the Tree Bumblebee, Bombus hypnorum.
It is quite common for it to nest in behind Fascia boards.
The actual nest is probably in the loft insulation.
Take a look at my article about this species which is tucked away in the About Bees area of the website.
Here is a quick link to the article :-
I hope this helps !