The V&A? The Science Museum? I’m sure, during childhood visits to the capital, that my parents must have shown me some of London’s biggest attractions, but the memory that’s really stayed with me is of something much smaller. We had called in at a Georgian townhouse late one dusky afternoon – and once inside were invited to climb up on to the roof. There, as evening fell over the skyline, I found a scaled-down railway station with a miniature train, steam pouring from its chimney, that skated over a pond and occasionally plunged into a tunnel. It was the stuff...