After Bill Turnbull was diagnosed with prostate cancer, he decided to document his bid to beat the disease in a film for Channel 4. Georgia Humphreys hears about the emotional process of making Bill Turnbull: Staying Alive.

Bill Turnbull can't help but feel a little embarrassed while watching his documentary about living with cancer.

"It's a bit of a blubberthon," confesses the former BBC Breakfast anchor, 63. "I think, 'Gosh, I wish I wasn't crying quite so much, or so often!'.

"But, the most satisfying thing is the reaction of the people who watched it; if they're moved by it, then we've achieved our purpose."

Guildford-born Turnbull first revealed he had advanced prostate cancer on The Great Celebrity Bake Off for Stand Up To Cancer last year.

Now, nearly two years after his initial diagnosis, the father of three - who married wife Sarah McCombie (known as Sesi) in 1988 - has chosen to share his story in Bill Turnbull: Staying Alive.

In the intimate, one-off film for Channel 4, we not only see what it's like to live with cancer, but also how he's doing anything he can to learn how the disease can be staved off.

What reservations did he have about doing this project?

"If you're going to make a documentary about yourself when you've got an incurable disease, you know you're going to be exposing quite a lot of your private life. And I suppose I did worry a little bit about how much we were going to put on show.

"But you have to be prepared to let people see what it's all about - there's no point in making the film otherwise. You can't sanitise it."

Currently, the journalist, who works as a presenter for radio station Classic FM, feels "pretty good, really".

He's on a new treatment, called Radium 223, "which is as radioactive as it sounds", to try to bring his PSA [prostate-specific antigen] levels down.

"It seeks out the tumours on the bone and attempts to destroy them"," he explains.

Prostate cancer is one of the deadliest cancers - it affects 47,000 men a year in the UK, and every 45 minutes, one man is killed by the disease.

But it's also one of the least known. And, as many others do, Turnbull ignored the early warning signs.

"There were familiar aches and pains, same sorts of ones coming back," he recalls.

"I had an aversion to going to see the doctor like a lot of men, because I didn't want to go to the surgery, 'There'll be other people there and they'll all have germs'."

Indeed, he hadn't been to the doctors for four years, and still felt he didn't need to; instead, he kept taking Ibuprofen to deal with the pain.

It was his son Will (he also has son Henry and daughter Flora) who finally persuaded him to go to see a GP. But, by that point, the cancer had spread from his prostate to his bones.

Treatment so far has included nine rounds of chemotherapy, and the articulate broadcaster doesn't sugar-coat how difficult he has found that.

"When you start, they tell you, 'You will have three bad days'. What used to sustain me during those three days - when you just don't want to do anything, you feel pretty awful - was I'd think, 'Well, it's only a couple more days'. But then with each round, I started to think, 'There's no coming out of this'.

"Three days became four days became five days... And I thought, 'Jesus, is this going to be like this forever? Am I ever going to feel better?'. And that's not a nice place to be."

One fascinating element of the film is when Turnbull visits a man named Jeff Ditchfield, who produces and processes his own cannabis oil, before passing it on - for free - to people who are suffering from cancer.

In doing so, Jeff risks up to 14 years in jail - and over the 15 years he's been making it, he has been arrested more than 20 times.

Turnbull notes that "there are at least 20 countries which, to a greater or lesser extent, have legalised marijuana or cannabis for medicinal purposes, which makes sense really".

"We need to be talking about it and examining it, because there's a potential benefit which we may be losing," he elaborates.

"So, maybe once everybody gets past talking about the 'B' word constantly in the halls of power, we can start doing more constructive things - this would be one of them."

CBD is a component of cannabis said to ease pain and anxiety; this is legal and sold in health stores, and Turnbull says he takes it "on a daily basis because it helps me sleep and it helps me with discomfort".

It's THC which is the psycho-active compound, and the illegal part, which causes a high.

But, "there may be other benefits to taking THC as well, you just don't know", suggests Turnbull.

A particularly memorable scene sees the presenter rather giggly after consuming cannabis oil with THC in - which Jeff has given him - through a vape pen.

"That was the one thing I did feel awkward about, more than the crying actually, because it was technically an illegal activity, really," he admits.

"So, there was that element to it - and looking like an idiot. But, on the other hand, when you've done Strictly Come Dancing [he appeared in the third series], you've kind of broken that bar already!"

Turnbull has been very open about his cancer journey on Twitter.

Asked whether he will continue to share information about his treatment on the social media site, he says: "I don't know - we'll have to see what happens. It depends what it is really. Who knows what's down the line?

"I'd really love to be able to tweet one day, 'Hey I'm cancer free', but I don't see that happening, just at the moment. But you never know.

"And those are four very important words - 'but you never know'. Because there's always possibly something around the corner."

Indeed, this sense of hope and positivity really shines through in the documentary.

It's an attitude reflected in real life by the calm, affable and witty Turnbull.

"I go down the bottom of the garden sometimes, usually at night-time, and I shout at my cancer and tell it to get stuffed - I use stronger language - but I say, 'Not today, you're just not having me'," he divulges.

"I don't believe in fighting cancer, but I believe in keeping it in its place. That's just my view."

Bill Turnbull: Staying Alive airs on Channel 4 on Thursday.