I first got interested in angels when researching my dissertation for my theology degree. I put articles in the Christian press and local church magazines and got 90 replies from people who wanted to talk about their experiences.
Local press coverage led to more people coming forward, and I broadened my research to include other religions. I have had more than 800 letters from all types of people, including doctors, barristers, the unemployed, prisoners and a professor.
I am not trying to prove that angels exist - I have never seen one - but I want to explore the phenomenon. For some, an encounter with an angel can be a life-changing experience: one woman in my BBC programme Angels tells of how she felt feathers behind her legs and a force lift her in the air when a car was hurtling towards her.
Christians, Muslims, Jews, Buddhists and Hindus have angel experiences. They seem to cross cultures and faiths. I think people believe they have the angel encounters they talk about. The language they use and the narrative style is very vivid: although it may have happened 15 or 40 years ago, it is as real to them as if it happened yesterday.
I am interested in the effect the experience has on individuals - such as removing their fear of death or opening them to new experiences. One woman who had been blind from birth "knew" what a smile looked like after seeing angels surrounded by light in a train compartment when travelling to see her sick father. Such angel experiences of blind people confound psychologists who explain angels as mind projection.
I am also interested in the sociological value of angels in a secular age. For many, traditional belief systems have collapsed. They are the New Age "spiritual shoppers", and a belief in angels can fit into a pick-and-mix approach to looking beyond the material world. You do not have to repent or accept a complex belief system to believe in them.
Angels have always been with us, but popular culture is now awash with them. The phenomenon started to snowball in the mid-1990s in the United States and it seems to have more longevity than fads such as alien abductions and out-of-body experiences.
Angels are benign and they do not hurt anybody. In the US, angels are still very big, but they face competition from witchcraft. If that takes off in Britain too, it will be a lot more controversial than angels.
Interviewed by Helen Hague
Angels will be broadcast on the BBC's Everyman programme at 10.35pm, December 12.