A larks head know would never be used to secure a rope using in a suicide because pulling on only one end of the rope will make the knot slip. Dodds was correct when he used a hitch knot.
The considerable friction of a rope over a wooden beam would have made it virtually impossible for a small elderly woman acting alone to haul up another woman of similar weight and hang her.
When McDonald& Dodds come out of the Mara retreat, and DS Ross approaches them (with the dead body in the flat information) McDonald tells Dodds to "get an Uber", when the police car (carrying DS Ross) is in clear view.
Dodds finds a receipt under a bed in the victim's room and picks it up wearing gloves. McDonald then grabs it from him with bare fingers, obliterating any chance of forensic fingerprints.