Antiques Roadshow expert Mark Smith was tasked with valuing a Victoria Cross medal in a classic episode of the BBC show, which will air on Sunday.
As the team returned to Portchester Castle in Hampshire, Mark began: "This is a unique moment on the Antiques Roadshow. In all the years I've been on the roadshow, we have never had a Victoria Cross. The highest award for gallantry this country has. Who was this man?"
"This was my wife's grandfather. Unfortunately, I wasn't privileged enough to meet him but my wife knew him when she was a young child," the guest replied and his wife added: "I have very vague memories because I was only coming up for my fifth birthday when he passed away. I'm very proud of him obviously with what he achieved."
It was explained the Victoria Cross was awarded to the guest grandfathers for the Battle of Passchendaele during the First World War.
The guest said: "He took charge when he saw that the platoon he was with come out under intense machine gunfire. A lot of them got cut down including the commanding officers, he rushed forward and attacked German machine gun posts."
Mark read out the citation, which is what the King would have been told before awarding the medal.
He said: "In the attack on the morning of October 4 1917 east of Langemarck, Sargent James Ockenden was acting Company Sergeant Major, noticing the right platoon held up by an enemy machine gun which was causing many casualties. He, with absolute disregard for his personal safety, immediately rushed the gun, killed two of the gunners and followed and killed the third who was making his escape across no man's land.
"He then led a party to the attack on a trench. This party was heavily fired on as it advanced, Sergeant Ockenden dashed ahead and called upon the garrison to surrender. They, however, continued to fire and, with great boldness Sergeant Ockenden, then opened fire himself. Having killed four of the enemy, he forced the remaining 16 to surrender. During the remainder of the day, Sergeant Ockenden displayed the greatest gallantry for that he is awarded the Victoria Cross."
Mark explained the Victoria Cross is a "very humble medal" and Queen Victoria said, 'It's not about the medal, it's about the deed behind the medal.'
"She never wanted it to be worth any money, she wanted it made of a very cheap piece of metal, it's made of bronze. He also has a military metal. So that's another gallantry award, and I understand that also was for attacking another machine. On posts. It also was a Gallipoli. Not only was he at Gallipoli, but he was in the assault wave on the very first day of the attack up that beach
Mark said: "As we say, Queen Victoria, she didn't want it to be worth money, they do have a value. Any idea what they might be worth?"
"Not really. It's something that will never leave the family," the guest replied.
The Antiques Roadshow expert revealed the medal was worth £250,000 to £300,000 leaving the couple understandably speechless.
The guest replied: "I didn't think it'd be that much," as the expert added: "Easily."
"I need a stiff cup of coffee," the guest said and his wife remarked: "I'm a bit like my husband, shocked. I didn't expect that."
They explained they would be returning the medals to the bank and keeping them in a safety deposit box.
The expert concluded: "There is a man who, in seconds lived through some of the most awful things that you can ever think of and not only did he do his best, but he actually shone above all of his comrades.
"And, in so many cases he was there to help save people from disaster and certain death. But also he did all of that knowing that probably the answer for him was certain death, but yet he never even hesitated."
