It is a great pleasure to speak at this meeting since it gives me a chance to acknowledge the great influence Dennis Sciama has had on my life. It was Dennis who first introduced me to relativity as an undergraduate at Cambridge in 1968 and it was through a popular lecture he gave to the Cambridge University Astronomical Society in that year that I first learnt about the microwave background radiation. I well recall his remark that he was “wearing sackcloth and ashes” as a result of his previous endorsement of the Steady State theory. This made a great impression on me and was an important factor in my later choosing to do research in Big Bang cosmology. When I was accepted as a PhD student by Stephen Hawking, I was therefore delighted to become Dennis' academic grandson. (Incidentally since Stephen has related how he had originally wanted to do his PhD under Fred Hoyle, having never heard of Dennis, I must confess - with some embarrassment - that, when I applied for a PhD, I had never heard of Stephen!) The subject of my PhD thesis was primordial black holes, so it seems appropriate that I should talk on this topic at this meeting, especially as Dennis was my PhD examiner.