A top detective who was part of the team that brought the BBC's Sixth Commandment killer to justice says the case was unlike any other.

DCI Mark Glover was part of the unit that investigated "cold and calculated" Ben Field over the mysterious deaths of two retirees in Maids Moreton, Bucks.

Field was eventually found guilty and jailed for life with a minimum of 36 years for the murder of 69-year-old Peter Farquhar.

ALSO READ: What happened to Ben Field? Murderer of Peter Farquhar

He duped the retired English teacher into changing his will and then tried to make his death look like accident or suicide.

Field was also accused of plotting to kill 83-year-old Ann Moore-Martin, an ex-headmistress, but was found not guilty - although he did admit attempting to defraud her.

His heinous crimes are currently being dramatised in the Sixth Commandment, a hit BBC drama - which also features DCI Glover.

The former Thames Valley Police officer has revealed Field's case was different to the other murders he probed during his 30-year career.

He said: "This case was very different in that in most murders, you know what has happened - you just have to prove it.

"But, in this case, you had a hypothesis as to what had happened, but no proper evidence at that time to support it.

"You had a feeling but not all the evidence to back it up. It was frightening if our hypothesis was correct."

Mr Farquhar's 2015 death was initially thought to be an accident or suicide, and Field - with whom he was in a relationship - was not suspected.

But police became involved after the death of Ms Moore-Martin - and realised Field had become involved with both.

Officers later found a staggering trove of notes relating to both Ms Moore-Martin and Mr Farquhar.

It emerged Field seduced Ms Moore-Martin and she gave him thousands of pounds - which he claimed he needed to help his brother.

DCI Glover said: "[His notes were] a bit of everything. It wasn't in chronological order like most diaries - it was more like a notebook.

"They were all over the place and didn't necessarily run chronologically.

"They were his thoughts, ideas, what he had done - all those kinds of things.

"The amount of material we seized in house searches was phenomenal. It's not just what I call physical material, but digital material.

"Not only did Ben keep diaries himself but he typed everything out on computers so we had a mass of digital information to download and go through."

Field was arrested in 2019 - and DCI Glover said it became clear the killer was a narcissist.

He said: We called it the 'Ben Field roadshow'. Whatever he did, he tried to take control and dominate.

"He was softly spoken and polite, and all the things that people found attractive about him.

"It was all about him trying to take control of the situation and us trying to take the control away from him and controlling him in some way."

Field was eventually convicted and jailed.

A court heard he carried out a sustained "gaslighting" plot aimed at making Mr Farquhar question his sanity.

The victim's drinks were topped up with bioethanol and poteen, a high strength Irish alcohol, and his food was laced with drugs.

Jurors were also told Field "suffocated" Farquhar when he was too weak to resist - killing him.

It was said Ms Moore-Martin had died of natural causes, but Field had defrauded her.

Despite devoting months to case, DCI Glover said the investigating team did not celebrate when Field was sent down.

He retired in May 2018 in the midst of the investigation, but returned as a civilian investigator.

DCI Glover said: "It is funny. You are always relieved to get the conviction and that the jury has agreed with you.

"People often think you go out and party to celebrate but you don't you feel quite flat after it.

"There aren't any winners really, Ben Field has gone to prison for 36 years but there are two families who are devastated by it.

"Two deceased people, one of which he was found guilty of murdering. It is good to get the conviction but a very strange feeling afterwards."

The story is now being retold by the BBC in the Sixth Commandment.

Timothy Spall plays Mr Farquhar, Anne Reid plays Ann Moore-Martin and Jonathan Aris plays DCI Glover.

The former detective praised the details of the production and says he hopes the series educates the public about the warning signs of manipulative and controlling behaviour.

He said: "The biggest thing about it is education because everybody thinks that elderly people are vulnerable through physical or mental health.

"Whereas actually, this highlights that they can be lonely for a whole load of reasons.

"Sexual orientation, religion, all those kinds of things can make people vulnerable as well.

"I really hope and like to think that came through and people are sat at home thinking about their own relatives and things like that.

"Ben was 26 when he was in a relationship with Ann who was 83.

If there are people at home sat at home thinking about relationships that they know of and start questioning it and some good comes of it then that's a positive."