A recent inspectors' report highlighted Merton's success in getting excluded children back into school: its reintegration rate of 34 per cent of children moving from pupil referral centres into schools is among the best in the country. Much of that success is down to work with families, says Mr Horsford. Even before a child is formally excluded he tries to sit down with the family and plan the future, asking them to name their preferred next school and - unless there is good reason not to, such as giving several excluded children separate fresh starts - promising to try to obtain it.
"Unless the parents are on board and feeling positive about the new school, it's not going to be successful," he says. "We have to reinstate their faith in education."
Gaining the support of angry or anxious parents can take time. Keith Shipman, inclusion manager for Merton, recalls the case of a boy who missed all of Year 7 because of illness. The following year the authority attempted to persuade his parents to send him to the borough's medical pupil-referral unit. (Merton calls its PRUs Smart Centres).
They were invited to visit; then the child was offered a place for an hour a day. "The parents were very protective, very fearful," says Mr Shipman.
"We said he could have a cab if he couldn't get there on his own. So we had a cab going across the borough and back, to get in him for an hour a day."
Gradually the child stayed for a morning, then for lunch, then the authority suggested he transfer to a unit for vulnerable pupils in a mainstream school. Again he started an hour a day:
"Then we said, 'Try going into history lessons, because you like history.'
We arranged for someone to walk him from the unit to the history lesson and back again.
"It's small steps, over a long time. You have to have a very good relationship with the mainstream school, because they need to understand he can't go into a class of 30 straightaway. But most of all you need the family's trust."
Trust is also the keyword when the authority proposes a pupil to any of the borough's eight mainstream secondary schools, two of them Roman Catholic, says Keith Shipman. Over the past 18 months the authority has created a reintegration panel on which every school is represented, and agreed a joint admissions protocol with all the schools on the basis of parental preference. (It has central admissions for all but the two Catholic schools) "Having the panel means heads are now much more aware of their collective responsibility," says Mr Shipman. "Every half term they get a list of who is in the Smart Centre, of which school the family wants. The pupil might not move for months, but it gives everyone a sense of destination."
That structure would not work without trust between the authority and schools, and trust between the schools that everyone will play by the rules, he adds. "A critical part of making this work is schools believing that we wouldn't place a pupil in the school if we didn't think it would be successful. The school has to feel comfortable. We'll tell the school everything, because at the end of the day we have to maintain a good relationship with them. If we put a pupil in there who isn't ready, we'll never get another chance."