A study in the current issue of the British Journal of Sports Medicine showed that of 31 elite athletes at the Sydney 2000 Olympic Games who presented to medical services with back pain, 58% had a prolapse of the lowest lumbar disc on MRI scan.
Lots of people are walking around with disc degeneration and are completely unaware of it. The findings on imaging correlate very poorly with symptoms.
On the other hand, running does put extra strain on the spine, especially if your technique or biomechanics aren't perfect.
Jez, don't put your running shoes in the recycling bin yet. But if you do decide to stop running, you're the sort who will soon fill the vooid with something you'll enjoy just as much. Good luck!