Unlikely allies Sarah Williams and Katrina 'Kit' Walsh egged each other on in a twisted game to plot the perfect murder then celebrated their killing spree by singing along to Abba musical Mamma Mia.

Williams, 35, was described during the trial as ‘jealous and obsessed’ after falling for Sadie Hartley's partner Ian Johnston.

Her best friend's diaries record how the pair plotted what they believed was the perfect murder to dispose of her love rival and revelled in the plans as 'the stuff of spy novels.'

The friends had met through a shared love of horse riding when Williams, of Treborth Road in Blacon, was just 12 and despite a 20-year age gap developed a relationship so close other friends described it as 'unhealthy'.

They were very different people. Walsh, of Hare Lane in Guilden Sutton, was intelligent, artistic and eccentric but with limited social skills always wearing a bandana to hide her alopecia. Williams had been bullied over a lazy eye as a youngster and was drawn to older people conducting a string of relationships including one with a 'sugar daddy' 40 years her senior.

The odd pair lived in each other's pockets and acquaintances described how they holidayed together and watched Harry Potter movies in Walsh's bed.

When Williams decided to eliminate her love rival she turned to her eccentric best friend, who she had supported through a difficult marriage break-up.

They worked together during each stage of planning the murder. They travelled to Germany to buy the stun-gun (taking a sight-seeing detour to look at the final resting place of a Celtic prince during the trip.)

Walsh bought the tracker that was put on Ian Johnston's car, as well as clothing and the car used on the day of the murder. She bought the murder weapon, a knife, using her Tesco Clubcard.

Sadie Hartley murder: Chester 'bunny boiler' love rival and twisted pal GUILTY of brutal stun gun and knife killing

In the aftermath of the murder, Walsh was instrumental in the clean-up and disposal of evidence.

Witnesses said the older woman was ‘in awe’ of her young companion, with CCTV recovered by police consistently showing Williams striding ahead, with her slightly-stooped friend following literally in her wake.

But the pair turned on each other in court both claiming the other must be responsible for the horrific bloodshed at Ms Hartley's home.

Sitting at opposite ends of the dock throughout the six-week trial they never acknowledged each other.

It was at the Chill Factore ski centre in Trafford in October 2013 that Williams first met Ian Johnston, 57, an ex-fireman who was an expert in the specialist field of Telemark skiing – a combination of the Alpine and Nordic disciplines.

They quickly began a sexual relationship but Mr Johnston continued to date Ms Hartley. He says that from Spring 2013 he tried to end the relationship but Williams became ‘possessive and difficult’ and refused to accept it.

That summer Williams sent Ms Hartley a letter revealing their affair but it failed to split the couple and in November 2014 Mr Johnston moved into her Helmshore home.

Watch how police pieced together Chester killers' 'premeditated assassination' of Sadie Hartley

However unknown to Ms Hartley Mr Johnston remained in contact with Williams. They continued to exchange explicit text messages some containing naked photographs, until December 2015, just weeks before the killing.

She told the jury she endured ‘a bad start’ to life when aged 13 she was dragged off her bike near the centre’s stables, bundled into a car boot and driven away. Her abductor then physically assaulted her before she was later abandoned on a country lane six hours later, she said. Her barrister put forward the theory that it could explain her later mistrust of men and her irrational behaviour.

She left secondary school in Birkenhead. Merseyside, aged 18 and started looking for work.

Williams was already in a relationship with 57-year-old David Hardwick, who ran his own heating oil business. They had met when she was 17 at Wirral Riding Centre in Neston.

From 2005 to 2013, she sold personal insurance from the offices of HBOS bank near to Chester city centre. In 2006, she bought her first home in Victoria Road, Ellesmere Port.

Largely bankrolled by Mr Hardwick – who was still living with wife Rowena - she lived beyond her means thanks to his weekly transfers of £320 to her bank account. In June 2010 she moved into her current address at Treborth Road, Blacon, Chester, with Mr Hardwick giving her £75,000 to cover half the purchase price.

More cash flowed into her pockets as she rented out her Ellesmere Port property and later acquired another property in Leasowe, Wirral, to rent out thanks to a £43,000 ‘loan’ from Walsh.

Williams and Mr Hardwick’s life revolved around horse riding and the many exotic holidays around the world they would enjoy - up to 12 a year, according to Mr Hardwick.

Just before her 30th birthday, Williams was driving on the M60 motorway in Manchester when she glanced across to the highly visible Chill Factore indoor ski centre and thought ‘I fancy a new hobby’. Mr Hardwick said he was too old to ski but faced with the prospect of her going alone – and maybe fearing losing her – he tagged along.

She threw herself into the new pursuit with gusto and quickly became an accomplished skier. Williams and Mr Hardwick spent so much time at the Crystal Ski travel agent’s at the Chill Factore that they eventually gave her a job as a sales advisor.

Within months of first learning to ski Williams was having an affair with her instructor - 47-year-old married Thai martial arts gym owner, Somapat Sitiwatjana. Mr Hardwick initially sanctioned the fling, said Williams, but she later thought he was on the verge of leaving her after he became jealous.

He stayed with her though and after Williams’s relationship cooled with Mr Sitiwatjana – known as Master A - she moved on to sleeping with another ski instructor.

Williams met Andy Poole while staying in the same ski lodge on holiday with Mr Hardwick in the French resort of Tignes in January 2012.

A sexual relationship developed on their return to the UK, apparently behind Mr Hardwick’s back, before Mr Poole said it finally ended 12 months later because Williams would not leave her elderly lover.

Williams swiftly turned to her next conquest, Mr Johnston.

Their liaisons culminated with a spurned Williams writing to his new partner, Ms Hartley, accusing her of buying Mr Johnston’s affections and boasting about their sex life.

Her desperate and irrational attempt to break the couple up ended in failure and a bitter Williams meticulously planned the ruthless execution.

Horse riding instructor Walsh did her best to appear a ‘zombie’ and ‘the caricature of a fool’, jurors were told. On arrest Walsh was designated an appropriate adult due to her behaviour and gave a bizarre, trance-like, stream of consciousness commentary, telling detectives "I’ll follow the memories" as she led them across farmland where she had buried the stun-gun and other evidence.

But as with her claims of a poor memory, it was all an act, the jury were told.

Later she told police she thought she was playing out an elaborate charade with Williams, like in the TV show Hunted.

Heavily tattooed on her arms, Walsh was in a biker group and rode a Harley Davidson she called Raven. She owned a horse called Zephyr, used Tarot cards and had a penchant for making jewellery, especially dragon-themed pendants.

She married Kevin Walsh in 1984, sharing common interests in horses, motorcycling and even Viking re-enactment events, but he left her for another woman in 2008.