bluebelle I think Roz's husband is the killer. His car was seen pulling up and in the vicinity of Tim's flat around the time of the murder. The nurse living downstairs confirmed it was a white guy driving (she was expecting her own taxi so noticed it).
Hub knew Ros was in there, watched her leaving, got himself in somehow, and killed him.
Or, Ros had already killed him, by accident, panicked and left.
If it was Hubby, he cleaned up, put Tim back into his day clothes, cut off his fingers, took the laptop and the rest of the evidence, put it into his boot and drove home.
When we see Ros going into the boot of a car, we assume it's her car. But it's Hubby's. This explains why she breaks down in the garage. She realises what's happened. She then uses the evidence to plant, to cover for her husband.
Later, we see a shot of Ros's car boot, empty and clean; because it never had the evidence in it!