Durham is a very good uni but perhaps it's 3rd tier. Durham suffers from the fact that it is up north and many southeners do not wish to go that far away from family. It also lacks the connections available for UoL unis which are based close to city. In terms of rep it is perhaps inferior in terms of alumni to sum of the UoL unis. But most importantly it lacks international recognition compared to the likes of oxbridge and LSE.
In spite of this, Durham is still a fantastic uni and ranked in top 50 Europeen Uni's. (nb. i am being particularly critical as you asked why durham isnt raved as much on TSR as sum other uni's.)
You've roughly got:
1)Oxford
2)Cam
[gap]
3)LSE
4)UCL
[gap]
Manc
Notts
Bristol
Durham
Kings
Warwick
[gap]
the rest.
(the 3rd section is in no particular order.)