But you said "The most comprehensive annual research is the Adoption UK Barometer report and this year, about 40 per cent of adoptive parents said they had considered returning their children to care."
My point is that the Adoption UK barometer is not formal research. It may well be that families who are struggling are more likely to complete it.
I never completed these because I never had the extreme challenges and I was very aware that it would be detrimental to those who do.
The other big issue of course is age. Plenty of adopters have a wonderful time when the children are younger, then it goes massively wrong when they get to secondary school.
Imo the Adoption UK barometer is interesting and shows trends but self selection means it's hard to be clear on the numbers.
Potato Group has done some interesting work recently too. Have you seen that? They would claim to represent mainly those with the extreme challenges.