The ex-cop claimed he acted in self-defence when he allegedly assaulted an inspector catching him having an affair with his wife.
Gavin Harper, 45, found Stephanie Glynn, 40, and Andrew McLullich, 42, in a Screwfix car park in Birkenhead, Merseyside, late on February 16, 2021.
He said he didn't go there to be violent, but to film the Merseyside Police colleagues and report them for having an affair during the pandemic.
Harper said he told the pair 'I've got you now' before being hit and knocked down by the inspector, who he claimed hit him again in a 'rage' for 'making derogatory comments about his penis' while his wife – partly naked – kicked him.
While testifying in court, he told jurors: 'That's probably the worst thing about it.
'I was thinking how has my life gone from being happily married to being on a Screwfix car park having the s**t kicked out of me by my wife.'
The security officer is on trial for unlawfully wounding Mr McLullich and stalking Ms Glynn for two months before the incident.
Prosecutors say he couldn't cope with the end of his marriage and attacked Mr McLullich after becoming 'obsessed with catching them in the act'.
Harper said he first became suspicious about the pair when he noticed his wife's car phone call history had 'lots of calls' from Mr McLullich.
He said Ms Glynn told him she was his mentor at work and he was helping her through her police sergeant's exam. He told the court: 'I knew something was going on but I couldn't prove it. I would repeatedly ask her and she said 'it's in your head, it's in your head'.
Harper said he decided to activate the microphone on the vehicle tracker and move it from the boot to underneath the driver's seat, allowing him to listen in real-time from an app on his phone.
He said one conversation between the pair confirmed his suspicions.
Harper said: 'They were talking about a previous night shift when they had met up and they had some sort of sexual activity, and they were laughing about who was going to sit in the wet patch of the police car.
I was upset. I now knew something was going on but I didn't have concrete evidence to confront her because she would just deny it.'
He went on to download her Snapchat phone history onto his device and found an 'absolutely massive amount' of messages between the pair, he said.
Harper said he used his ex-wife's phone to call Mr McLullich.
The defendant said: 'He said 'hi gorgeous' and I said 'it's Gav, Steph's husband'. There was then a pause and then 'oh s**t'.
He asked me not to tell the police or let them know what was happening, suggesting he and Steph would lose their jobs.
He pleaded with me not to tell his wife or ruin his children's Christmas.
Harper stated that Mr McLullich denied sleeping with Ms Glynn but admitted to exchanging inappropriate messages and videos, acknowledging they were disgusting.
The accused took a risk by assuming the pair would be at the car park, using past tracker data from his ex-wife's car and their work schedules.
He mentioned that his ex-wife knew about the installed tracker for security, but clarified that there was no tracker on the night of February 16.
When testifying on Tuesday, he informed his barrister, Suzanne Payne, that he didn't drive to the car park with the intention of fighting the inspector.
He stated that he didn't plan to be violent.
Ms Payne asked, 'What did you believe you needed to do?'
Harper replied, 'To obtain undeniable proof of their affair by filming it and providing the evidence to the police.'
Ms Payne inquired, 'For what purpose?'
Harper responded, 'To show that two serving officers were having an affair during Covid.'
He explained that as he approached Mr McLullich's car, he saw that he was clearly having sex with someone, and added that he looked through the window and saw him having sex with Steph.
When asked why he opened the rear passenger door while filming on his phone, he mentioned, 'To capture better footage, I suppose, to ensure I had undeniable evidence of their actions.'
Ms Payne asked, 'Was anything said?'
The defendant stated, 'I can't recall exactly what I said, but it was something like “I've caught you now”. Steph screamed and said it's Gav. I was upset but strangely relieved.'
Ms Payne asked, 'Were you angry?'
Harper, from Liscard, Wirral, said, 'No, quite the opposite.'
He mentioned that Mr McLullich started punching him repeatedly.
Ms Payne asked, 'Did you try to defend yourself?'
Harper stated, 'I did hit him – three, four times.'
Ms Payne asked, 'Did you continue to defend yourself?'
The defendant responded, 'No. I didn't have the energy for that. I wasn't there for that.'
He then mentioned that he was 'leg-swept' by the inspector and punched again while on the ground.
Harper mentioned that his wife, who he said was naked from the waist down except for her socks, told Mr McLullich to 'get his phone'.
He mentioned that during the struggle, Ms Glynn then 'started kicking him'.
Harper mentioned that he later took out his phone and started taking lots of still pictures.
The defendant mentioned, 'I made a derogatory comment about his (Mr McLullich's) penis. He didn't like that. He became enraged and punched me a couple more times in the face.
'Steph got between us and he stopped it.'
The trial continues on Wednesday.
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