Far-right leader Oswald Mosley was leading supporters in preaching hate against minorities, which resulted in attacks on Jews, their businesses and their homes.
Describing his experience, Mr Block explained: "It was the bloody Nazis on the streets, and we'd just finished fighting them. The British Government allowed it to start. They allowed these people to make the same Nazi speeches they did before the war.
"I'd just done four years in the RAF. I saw my friends killed. I started with 84 guys and two of them survived the war.
"It was ridiculous to see British fascists standing on platforms waving their arms in the air saying: 'Heil'.
"How do you think I felt? Wouldn't you do the same?"
Mr Block took up the post of central London area commander a few months after the 43 Group started, leading ex-servicemen and women through a series of violent confrontations with the fascists.
"The furthest we went was arson and grievous bodily harm (GBH)," he recalled. "We had members arrested for fighting. A friend of mine was in the Old Bailey for GBH and was found not guilty."
Mr Block's friend was not the only one put on trial. In 1948, activists Gerry Flamberg and John Wimborne were acquitted of the attempted murder of British Union of Fascists member John Charles Preen.
Mr Block continued: "There were arrests every week. I got arrested one night at a Church of England meeting we knew the fascists were going to attack, with 20 or 30 others. There were so many of us the sergeant threw us all out the police station."
Now aged 90, the Stanmore resident and father-of-two had no doubts that the violent tactics the vigilante group used were necessary.
"There was a bookshop that was selling fascist books, and we made sure that bookshop didn't function any more," he said.
Did someone set fire to it?
"Somebody might've burned it down. They might've done, yes. That's how you stop these people."
The group clashed with the Board of Deputies and even broke off relations with the Association of Jewish Ex-Servicemen and Women (Ajex), but Mr Block has no regrets.
"We stopped them getting into halls, lectures, schools - they couldn't give talks because we caused so much havoc. You can't hold a meeting when there's a disruption, when someone sets off a smoke bomb.
"It was very important, what we did. It stopped them getting a foothold."
Mr Block said he had no time for civil liberties when it came to poisonous, damaging principles being espoused. "You can talk about democracy and all that crap, but you've got to be reasonable about it. It's all right having freedom of speech - unless it affects you."
And if the fascist groups had become more popular, more powerful and more violent, then the 43 Group would have followed suit, he said.
"What we did was right for the times, but if things had progressed, we would have had to progress like everyone else, with bombs and guns and things like that. All of our guys were weapon trained, so if you put a machine gun in their hands they would have known what to do with it."
What had his family made of his vigilante activites? "My grandfather used to wonder where I was going at night, but I didn't tell them much about it because they'd have been upset or worried."
The 43 Group disbanded in 1950 and Mr Block said his daughters had never asked about his experiences.
"You can only know the importance of it by being there at the time," he explained. "I lived a normal life, as a musical technician. This was a thing you did when you were needed, but after that things became normal."
The goal was survival, and it had been achieved. The authorities called the 43 Group an anti-fascist organisation but Mr Block says, "I don't know what fascism is. I'm not a politician. As far as I was concerned, they were antisemitic, and I'm an anti-antisemite.
"We did what we had to do."
