Greybeard (1964) by Brian Aldiss.
I read this years ago, or tried to but I didnt get it at the time and probably didnt finish it back then. This time however I found it very enjoyable!
Basically its set in about the 2030s, 50 years after a nuclear accident when bombs were set off in space, causing a catastrophic disruption in the Van Allen belts that surround the Earth and protect us from solar radiation.. The 'accident' resulted in this radiation from the sun briefly reaching the Earth, rendering the human ace sterile. At the time the book opens the human race is represented by the elderly, eeking out a living pottering around Oxford and London, looking for, and on guard against, others. There are rumours of new children born but it seems to be all myths perpetuated by deranged old lunatics, or is it?.....
Re-reading after all these year I would heartily recommend this if you are a fan of post-apocalyptic works- in fact I'd go so far as to say this is the best Aldiss book I've read so far!