I was called up for my first at 48 (there was a trial, apparently but it was not made clear that this is what it was), second at 51 and third at 53. This is because they recall every 34 months rather than 3 years. I was recalled after the second one for a patch that looked like cancer, and had lots of mammograms and ultrasound, but it was the result of using only two views in the screening - they sometimes come together to make something that isn't there, apparently.
I feel a bit overscreened. My friend, however, found a suspicious bit and was diagnosed with cancer just before she was 53, but they had not called her up at all.