might be some Melanotus ??

Stenagostus rhombeus usually has a V-shaped pattern in the hairs towards the end of the elytra, which I can't see in this photo, and I must admit this does make it look like Stenagostus rufus. However, that species has not been found in Britain as far as I know, and S. rhombeus does have a form where the V-pattern is not present, so I suppose this is more likely to be S. rhombeus, although it's difficult to be 100% certain. At 17mm it the correct size for S. rhombeus and a bit too small for S. rufus.

Do you have any other photos (a view from directly above would be best), or did you keep the specimen?

The other possibility is that it isn't Stenagostus at all - the thorax seems a bit short for S. rhombeus, but it's tricky to judge that from the angle of the photo. I can't think of any other genus that matches it better.