growstuff
FullFact doesn't claim that 1:13 is the correct figure. It suggests a different hypothesis and comes to the conclusion that an accurate figure is unprovable.
I have no doubt that there are people living in London illegally. However, I have no idea how anybody could produce anything like an accurate figure. It would probably be possible in a small, isolated community, but London's population is huge and very fluid at the best of times. If anybody has any suggestions, it would be interesting to hear them.
Its a complete muddle.
For those interested, here is the Times Newspaper correction of the original story,
www.thetimes.com/comment/register/article/corrections-and-clarifications-the-times-x9nbft5tq
"January 27, 2025
We wrongly said that a study on behalf of Thames Water showed “1 in 12 Londoners is an illegal migrant” (News, Jan 23). In fact the study covered only 7 million people in specific “water resource zones”, not the whole population of London, which is closer to 9 million. 1 in 12 was the highest of a range of estimates for the proportion of “irregular migrants”, but these estimates included some people who are not illegal immigrants, such as those given indefinite leave to remain, as well as some British-born children of migrants with irregular status"
The Telegraph and GB news have not offered corrections.