For decades Jo Cameron has been cheerfully bumping, burning and bruising herself in all manner of mishap, yet she never stopped to ask why her injuries did not hurt.

She gave birth to both her children without once resorting to drugs; she laughs off offers of anaesthetic during dental work, and when she burns herself while cooking on the Rayburn in her Scottish Highlands home, often the first she knows about it is the smell of her own burning flesh.

Virtually nothing worries her. When a wayward white van careered into her car on a remote country road, leaving her upside down in a ditch, it was she, totally unfazed, who found herself comforting the driver.

It was not until Mrs Cameron’s sixties, as she was preparing for an arthritis operation on her hand, that she – or anyone in the medical profession – first suspected she was different.