When he was a young teenager, Cottan worked as a golf caddy for Tommy Hunter at the Edge Park Golf Club, which earned him the nickname "The Caddy". Induced into the criminal underworld at a young age, he was groomed into joining Central Police so he could act as their mole from the inside.
He was previously married to a woman who worked in the Forensic Unit, but they divorced due to his former alcoholism and gambling addiction. His nickname "Dot" is a reference to Dot Cotton, a main character in the British soap opera EastEnders.
Prior to the events of Series 1, Dot was recruited into TO-20 by DCI Tony Gates, who took a chance on him despite his problems with alcoholism and gambling. At this time he was close to being terminated from Central Police, as he already had two Regulation 15 Notice's on his record. He previously worked out of Hillside Lane Station.
A Disastrous Affair
Cottan is interviewed by AC-12 regarding the investigation into DCI Anthony Gates regarding laddering, accompanied by his Police Federation Representative DS Raser. It is revealed that he previously had a gambling addiction, and was given a final written notice for dereliction of duty because of it. He also got into considerable debt and lost his wife because of it. As it was Gates that assigned him to TO-20 despite his breakdown, which resulted in him getting back on his feet, Superintendent Ted Hastings accuses Cottan of abandoning surveillance early as a favour to Gates, which allowed the murders at Greek Lane to take place. After being pressured by Hastings and Arnott, who threaten to give him a red notice and proceed with a criminal conviction, he tells them Gates gave him the order to suspend the surveillance early.
Now a Detective Inspector, Cottan is seconded from the AC-9 Witness Protection department, into AC-12 by DCC Mike Dryden, in order to aid the investigation into DI Lindsay Denton and the 5th September Ambush.
Upon realising his cover has been blown, Cottan weirdly sends a text message to three contacts on his phone ('Removals', 'Talk', and 'Ride') whilst pretending to check his calendar. Lambert, one of the Authorised Firearms Officers guarding the interview room is one of the recipients, and upon receiving the text proceeds to gun down the other AFO guarding the interview room. He then shoots through the glass walls in order to allow Cottan to escape. Before the AC-12 Building can be locked down, Lambert threatens an officer in the lobby to let them out. Cottan is given a Glock 17 sidearm by Lambert, and the two run down the high street in Kingsgate towards the Moss Heath area where their pick up point is located.
En route to Moss Heath, the two are confronted by an armed DC Kate Fleming, who orders them to drop their weapons. Lambert is the first to respond, turning and aiming at Fleming, but is gunned down by her before he can pull the trigger. Cottan and Kate engage in a brief standoff, but he is able to escape around a corner when Fleming loses her clear shot. Upon arriving at the extraction point in Moss Heath, Cottan is again confronted by Fleming. The two engage in a longer standoff, with neither being fully willing to shoot each other. Fleming demands that Cottan surrender, but is then knocked down by a Range Rover sent to pick up Cottan, allowing him to get in and escape.
When Fleming is able to get to a better vantage point and shoots the driver of the car, it crashes into another vehicle, injuring Cottan. Disoriented and injured, he stumbles from the wreckage and is again confronted by Fleming, who begs him to drop his weapon. Noticing that a gunman in the back of the damaged car has also survived and is about to shoot Fleming, Cottan throws himself in the path of the bullets and is shot 3 times. Fleming then shoots the gunman. Before he succumbs to his wounds, Cottan records a dying declaration, in which he provides essential information regarding Patrick Fairbank and other high ranking police officers which are part of the organised crime syndicate.
Cottan is seen in video footage recorded by a police bodycam as DC Kate Fleming took his dying declaration in Series 3. The footage is to Superintendent Ted Hastings by Assistant Chief Constable Derek Hilton. In it, Cottan reveals that the name of the police officer in charge of the other corrupt officers in the criminal underworld begins with "H", before finally dying.
Upon closer examination of his recorded dying declaration, DS Steve Arnott notices an additional feature previously missed by investigating officers. In addition to blinking when then DC Kate Fleming says the letter "H," he is also seen making gripping motions with his hand, tapping four times. Arnott notes that this is likely Morse Code, and that Cottan was trying to indicate that the letter H is not necessarily the initial of a person, but a wider clue. H in Morse Code is four dots, which leads AC-12 to deduce there are (or were) four senior ranking members of Central Police in league with organised crime: Cottan himself, ACC Derek Hilton, Senior Legal Counsel Gill Biggeloe and the fourth man, now identified as DSU Ian Buckells.