What is the UK prevalence of women overdiagnosed with breast cancer by screening?

The review on breast screening estimates that every year about 4,000 women are ‘overdiagnosed’ with breast cancer. ‘Overdiagnosed’ means that as a result of breast screening a cancer has been identified and treated, but in the absence of screening it would never have threatened life and would have remained undetected in a lifetime. Some of these overdiagnosed women - but neither we nor they know which they are - will be advocates for breast screening when that is both misinformed and misinforming.

I am wondering what the prevalence of overdiagnosed women is. The review isn't particularly confident of their figure of about 4,000 women added to the ‘overdiagnosed’ balance sheet each year. As no one knows which women they are, we currently have to accept screening means that, alongside your inaccurately billed 'life saving' early cancer diagnosis you will get pointless cancer treatment. This doesn’t work for me, as you can tell, but others may come to a different conclusion re the balance of risk v benefit.    

At the far upper end of my estimate, 25 years of screening at 4,000 per year could mean there are 100,000 overdiagnosed women.  Assuming, as I did, that a) no-one has died in those 25 years and b) the screening programme uptake has been full, and constant, since the start, 100,000 is clearly a major over estimate. If  4,000 per year has been approximately true for about the last ten years, say, and there will not have been a huge number of deaths from other causes (they weren’t ever going to die from breast cancer) that makes c.40,000 with the addition of some other women still marching along from earlier years.

It’s probably easiest to just halve the 100,000 and forget thinking and my spurious stab at accuracy. It’s still a prevalence estimate of 50,000 women, equivalent to the population of a small town. They have two personal tragedies. One they know of - their diagnosis of breast cancer; the second one they don’t – that their good fortune in getting an early diagnosis is entirely illusory.

Can anyone come up with a less flaky estimate for the UK prevalence of overdiagnosed women, based on facts and proper epidemiology?