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Matt Gatward: Diving into the moving tale of Tom Daley and his dad

Tom Daley: the Diver and His Dad, BBC 1

There is a room in Tom Daley's house where he hangs his many medals. When the window is open, the breeze gently blows them into each other and they jangle like a wind chime. The noise will be just that little bit louder this week as he will have hung up two more golden gongs after his exploits at the Commonwealth Games in Delhi that reinforced his position as the star of British diving. It's weird then that he is barely the star of his own home. As the title of Tuesday's BBC documentary about him indicated, he has to share that honour with his dad.

Rob Daley has a brain tumour and the programme spent as much time dealing with how he was coping with his illness as how Tom was hitting the water. And quite right too because it made for an engrossing, moving piece of television and showed the wonderful relationship between father and son.

At first, the show seemed to be all about Tom and his diving. It opened with him discussing a recurring nightmare where he is at the 2012 London Olympics and as he dives into the pool a shark breaks the water and Tom disappears into its mouth. Not a great dream for a diver to be having – but at least he could laugh about it.

But it quickly became apparent that Rob would be a key character. Not just because of the tumour but because of how close his relationship is with Tom and the "banter" the pair enjoy. "He pretends to have undiagnosed Tourette's syndrome," Tom said at one point of his dad. "He is embarrassing." "Is that the same dad who drives you round everywhere, Tom?" his dad asked. "No that's the undiagnosed Tourette's dad," Tom responded.

As Tom's agent, Jamie Cunningham, put it: "You don't get Tom Daley if you don't have Rob as who he is." Which is a down-to-earth, straight-talking, West Country bloke, dedicated to his family.

If anything, the programme – which had an abrupt ending and would have benefited from being shown post-Games – felt too intrusive on occasion. It was access-all-areas as the film crew filmed the Daleys in the build-up to the Games and as Rob dealt with the effects of the tumour. We were with him as he had his MRI scan and saw the brain images on the screen and learnt that the tumour had grown, thus preventing him from travelling with his son to Delhi. "I may not see him next year if I do go," as Rob put it in an effort to lighten the mood.

But Rob admitted to having his darker moments. "The 2012 Olympics. Am I gonna be going to that?" he asked. "I think about [the tumour] on my own. Am I going to see my grandchildren? But because I'm always laughing and joking, Tom thinks I'm fine."

There was one lovely exchange between Tom's two younger brothers. When asked if it was amazing to live with a world champion, Ben, aged 11, said: "Not really." "Shut up, Ben," said Will, aged 14. "It's not though," replied Ben.

Not everyone is in awe, then. Nor were his grandparents. "It's a messy-looking dive really," Rose and David agreed when discussing his new somersault with twist dive. "It's not clean. It's scruffy." With family like that, it's no wonder high-diving Tom, who came across as a thoroughly likeable young man throughout the programme, has remained so grounded.