There are two types of people in life - those who care about others and those who don't give a damn.
I first met the Princess of Wales at Centrepoint when I'd just been appointed head of the homelessness charity. Among my many tasks was persuading her to continue supporting Centrepoint, and fortunately she agreed to carry on as patron.
In my dealings with Diana, you could always be direct with her because she 'got' what we were trying to achieve. She may have been a member of the royal family but wasn't part of the establishment.
To her, it was immoral that people had no choice but to live on the streets, and she felt everyone should take responsibility for tackling poverty which is the root cause of homelessness. The fact her life was one of immense privilege was neither lost on her nor something she took for granted.
At homeless shelters, Diana would kick her shoes off and immediately sit down to chat with the young people. There was no formality or needing her hand held. In fact, she'd shoo me out of discussions with service users so she could chat to them alone.
Her legacy was to draw worldwide attention to unfashionable causes, and therefore help break down stigma. And Diana made people feel heard - she spoke for those without a voice.
Imagine you're a young girl effectively abandoned by the state and made to feel worthless. Then the most famous woman in the world takes an interest in your problems, listens to what you have to say, and understands your desperation.
Her gift was using her humanity to speak to their humanity. That impact can - and did - transform lives.
There are those who turn their nose up at celebrity, and I have no time for those who hide behind their public persona or are self-important. But Diana made a difference by using her prominence to draw attention to the causes she supported.
However awkward or tough, she was prepared to discuss what was in those days the unmentionable or controversial, including HIV and landmines.
The Princess was accused of 'politicising' the royal family when she gave a speech I wrote on homelessness. Neither of us had thought it controversial, but some politicians were annoyed because it highlighted the lack of investment in programmes to tackle poverty. The outcome though was that youth homelessness rose up the public's agenda.
Mental illness and drug and alcohol dependency were also issues that the Princess of Wales brought to public attention through her work with Turning Point. It is partly down to her contribution that today more people feel able to seek the support that they so desperately need.
The last visit we did with Diana was to a shelter in Berwick Street with her sons. It was mobbed by the press which was undoubtedly alarming for the young princes, but I remember how calm and protective she was towards them.
The night she died I was at my mum's house. The phone rang at 5am - it was a friend asking if I'd heard the news. I hadn't and got out of bed straight away to switch on the radio.
Diana was always hands-on. Only a few weeks before we'd been in discussion with her office about her next visit. Her death was completely unexpected and the tragedy was that it robbed two boys of their mother.
Twenty years on, we need another Diana. In this time of austerity, when the divide between the haves and have-nots is greater than ever, we need someone who can persuade others to see the problems those on the margins face. Otherwise, the danger is that so many will get left behind.
We need someone with the same influence who is unafraid of drawing attention to inconvenient truths, and capable of making politicians and society think differently.
Someone who gives a damn.