Hello Satan Klaus,
I'd forgotten English wasn't your native tongue until you mentioned it at the end of your story. I agree with the others who have said you did a wonderful job and I have no specific criticisms or corrections that haven't already been mentioned. However, I would like to take this opportunity to discuss the following paragraph -- not as a criticism of what you've written, but for the purposes of discussing narrative writing in general.
I get some sense of the man in the scene, but it's still quite vague. You mention later that he speaks in a mannered 'Oxford voice', and that helped to clarify his image. However, the reference to his voice could have more impact if the visuals were more well defined to begin with. How to do this?
I'd make a list of things about this 'Oxford man' and his appearance:
Of indeterminate age, somewhere between mid-forties and early fifties.
Wears a dark, double-breasted suit over a crisply starched white shirt.
The shirt has cuff links.
What color tie? The character of a man can be expressed by his choice of tie. Red speaks of boldness; blue of quiet confidence and so on. An Oxford man might well wear a dark navy colored tie emblazoned with a small golden crest of some sort. "Old school tie", so to speak. Is there a matching kerchief in the breast pocket of his suit?
It's unlikely his shoes could be seen from the adjoining room if he's sitting at a table. His hands, however, would probably be resting on the table. I imagine him to be impeccably groomed and thus would have neatly manicured nails. Does he wear any rings? Perhaps an ornate gold band set with an onyx stone on his little finger. Does he wear any other jewelry such as a watch? If he does, is it an expensive looking gold watch or more a classic style with a leather band?
Are his hands lightly clasped in front of him on the table? This would suggest he's confident in himself. Or maybe he's absently toying with the ring on his finger?
What sort of face does he have? Are his features well defined, or is his face rounder and softer? Is his skin tanned or fair? Does he have any distinguishing blemishes?
You've mentioned a slight receding hair line and its salt and pepper coloring and broadly described him as a businessman. By this I would assume his hair is cut short though you could accentuate his 'Oxford-ness' by describing his hair as slightly longer than most men his age -- such as academics often wear their hair longer.
Obviously there are a lot of ideas to consider above, and I wouldn't use all of them at once. If I was to rewrite the paragraph I might begin:
The man sat straight-backed in his chair. He appeared to be relaxed with his hands resting one atop the other in front him on the polished steel table. The impeccable way in which he dressed seemed incongruous within the confines of the interrogation room and its graffiti covered walls. (Detective whateverhisnameis) leaned close to the glass of the observation window and quietly studied the man in the adjoining room.
You could at this point make reference to his suit or his physical features, but probably not both in the one paragraph. I've avoided mentioning his age also, though this could be addressed in the context of his physical features. What I wouldn't do is make any specific guess but rather, describe it as being either 'indeterminate' or relative in some way to the age of another character in the scene.
Those are just a few ideas off the top of my head. Hope they help
anonymouse