When the world's first state pension was introduced in Prussia in 1889, the qualifying age was 70 and the average life expectancy was 40. Half a century later, in 1935, many countries lowered the retirement age to 65, but still barely half the population lived long enough to claim it. Now, it's clearly a very different story.

With the help of PD James, Sarah Dunant looks at how the UK can tackle the demographic nightmare it currently faces - an ageing population but falling birth rates.

Producer: Adele Armstrong Sound: Peter Bosher Production coordinator: Helena Warwick-Cross Editor: Richard Fenton-Smith