After a retirement of just four and a half hours the eight man four woman jury returned to court to convict Groves, who bludgeoned Mr Martindale to death at a remote farm after a row over drugs.
Full horror of Billy Boy Martindale's death described in court Family of Billy Boy Martindale 'feel empty' after his lossGroves will receive a mandatory life sentence but the judge has yet to set a tariff of the number of years he must remain behind bars before he is even considered for parole.
He will also impose a separate sentence on Groves who was also convicted of inflicting grievous bodily harm on another man, Paul Meeking, during the incident. He had denied but was also convicted of that offence.
During just over a week of evidence and submissions by lawyers and the summing up by High Court judge, Mr Justice Sweeney the court was told of the fatal confrontation between the two men at Mr Martindale's smallholding, Meadow View Farm, Mill Lane, High Ongar, on 7 September last year.
The two men who were said at one stage to have been friends had fallen out over a drug dispute it was claimed..
The jury was told that Groves and Mr Martindale fell out over a drug dispute.
He claimed in court that he thought Mr Martindale was armed with a shotgun or machete and was going to shoot him and that he acted in self defence.
He told the jury he panicked and struck Mr Martindale with the pick-axe handle and then left, believing Mr Martindale was unconscious.
Groves claimed Mr Martindale had been threatening him telling him he had to grow cannabis crop for him (Martindale) because he had ruined one before and it had left him (Martindale) £2,500 out of pocket.
Groves said he had repeatedly refused to do it but eventually he decided to go to the farm to talk about it.
He told the jury he had gone to see Mr Martindale after he phoned him threatening to stab his (Groves') father.
Although told the jury he believed Mr Martindale was a "bully, gangster and a criminal", he said he went to see him to "tell him to calm down" to "chill out."
But he claimed that when he got there Mr Martindale was at a table with and he had thought it was a gun or a machete.
He told the jury : "I panicked and thought it was a shotgun. He had shown me a shotgun in the past and was always bragging about guns and things and quite passionate about them and it just looked like the butt of a shotgun.
"I panicked and punched him. I thought he was going to shoot me."
He claimed he was then attacked by Mr Meeking with a pick-axe handle, wrested it from Mr Meeking and hit him with it. He said he had then hit Mr Martindale twice and had left thinking that he was unconscious.
In his final summing up on Wednesday the judge warned the jury not to let themselves be influenced by speculation, emotion or media reporting.
He told them: "What is required of you is a cool, calm, careful and dispassionate consideration of the evidence together with the courage to return true verdicts based on that evidence whatever the consequences may be."
