Hello Tom,
welcome to birdforum!
Normally I would write, that this is a bird for quiz-modus, but I hesitate to write this, with a new member. I habve thought for other word to describe it, but as a non-native speaker, I am not good in this.
So I hope you are not offended, but this a the term I use, to explain, when a picture is to difficult for me to ger a sure ID. But there are others, who can jump in.
First impression: its a Finch or Sparrow.
Best looking Finch is a Greenfinch, they can look quite uniform coloured, but colour is the wrong brown for this species (but does the window results in a colour shift?). More, I would expect to see green or yellow-green in the wing and tail. And I got the impression that your birrd looks to compact and small for a Greenfinch.
So its either a Tree or House Sparrow. But which one? Folowing ponts are better for Tree imo:
-compact, small, short-tailed jizz
-little contrast between cap and flanks, both areas show a similar tone
-colour of the head is very good for a drab, pale Tree Sparrow, espacially when a picture through a window-effect is considered, that reduces contrast and adds a greyish overlay on the picture.
-shouldnt thr dark pattern and pale wing-bars be seen here, despite picture quality? I think, propably not, but I am not sure.
I dont see an European Robin, but its just a gut feeling.
Conclusion? I know, all the mentioned points cant be judged with certainty here, but I would say Tree Saprrow (or Greenfinch?), if I am forced to put a name to this bird.
Are there more pictures of this bird?