Is it not obvious...
...that Noel can very well be Denis? The story seems deliberately balanced for both possibilities to be credible (the "prima facie" Leullet helping Denis commit suicide, or maybe Denis ending up killing Leullet and taking his place.) The final Auden quote begs for it, as do the practically spoon-fed parallels (half-brothers, alcoholism...)
Parallels aside, Bonheur is ecstatic when she sees Noel on TV -- could it be it really is her companion-for-5-years-turned-bum Denis? Why not, seconds later she's speaking very fondly to Bellamy of the time spent with Denis in that apartment, just before telling Bellamy she's not only dropping all charges against the supposed-Leullet-killer-of-Denis, but that her lawyer friend will actually defend him. Also, more intrigue, after we are shown Mme. Leullet dead in disputable circumstances we learn that Denis was an ex-pharmacology student (a gratuitous revelation? Don't think so.)