As he waited at the bus stop William Grove, 84, was anxious about his civic responsibilities. He was due to be opening a community hall for a lunchtime gathering of his fellow pensioners. But as he paced the pavement his attention was drawn to a commotion outside the nearby jeweller’s.

Along with dozens of shoppers, Mr Grove saw the two balaclava-clad men smashing sledgehammers against the toughened glass windows.

“At first I thought it was a prank and that there were hidden cameras,” he said. “The sledgehammers were bright orange – but when I realised it was real I went over.” As scores of more able-bodied passers-by looked on, Mr Grove, who served in the RAF and helped to train the Indian Air Force, made his way towards the Ernest Jones shop in Richmond, southwest London, as fast as his legs could carry him.

“My plan was to carry out a move I had been taught in the Forces and wait for others to help. I was going to grab his head and put my knee in the man’s back before grabbing him round the chest, holding him until others intervened,” he told The Times. “It’s unarmed combat. You creep up behind somebody and grab them. One of them had his arm through the window and I knew then that he was partially disabled. There was no way he could swing the hammer and he had to be careful about getting a severe arterial wound. I was going for his head rather than his balaclava.”

But the raider’s balaclava came off in his hand and the would-be robber turned, preparing to attack his assailant, only to be shocked to find it was a grey-haired pensioner. The two men ran off, dropping their sledgehammers and leaving behind the balaclava.

“As soon as this happened I think they were both so taken by surprise at what I did that they just ran off. I didn’t have time to be scared, it was all over in about two minutes.”

The hero of the hour then turned to the gathering crowd, smiled, ambled to the bus stop and stepped on board: his crime-fighting cameo on Thursday was over. Mr Grove, a former civil servant, said he had to rush off as he was late for a meeting and, as he had the only key, he did not want people to have to wait in the cold.

Nick Thompson, a 35-year-old IT technician who works opposite the store, watched the drama unfold. “It was a busy day on the high street and there were crowds of shoppers watching. But these days people don’t want to have a go in case a robber has a weapon. Everyone was standing back when this old guy at the bus stop decided he was not just going to stand there and watch. He went over and started flapping at one of the robbers.

“He grabbed him from behind around his shoulders and managed to pull his balaclava off his face. The robber turned around and saw the old guy and just could not believe his eyes. He could not believe that somebody that age had taken him on. If it had been somebody younger, he probably would have whacked him.

“If he had hit the old man with the sledgehammer he could easily have killed him. The old man risked his life against someone much younger and bigger. He is a hero.”

But Mr Grove insisted that he was no hero. “They were just incompetent,” he said, before adding: “The bravery thing has been exaggerated, please play it down.”

And in what is probably the last straw for the robbers Mr Grove also walked away with a Rolex watch – courtesy of the jeweller.

Mr Grove said: “Well, I will probably raffle it. I have a perfectly good clockwork watch I inherited from my father, and it keeps perfect time.”

Police are looking for two black men aged between 18 and 20.

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