Sofiatheseventh - exactly right, if you get managed out of Thomas's, EH, etc your chances for dulwich, colet court, etc are rather minimal! Your best bet is probably broomwood / northcote lodge , but children already at broomwood lower school get priority at 8+.
I am choosing a pre prep with a view to staying there until 11+/13+, we are not planning to move out of London, so the only reason to change would be if it's the wrong fit for ds.
Lolaloves - I actually was under no illusion that these assessments at 3 are meritocratic ... I think it's a numbers game (boys vs girls, month of birth, nurseries, etc), so really not a case of looking for "the spark in their eyes" as they'll have you think ... these schools are all businesses, so I guess they have to have good relationships with the nurseries that send them most kids (which will be those around northcote rd, as that's where demand for Thomas's is highest), but still taking enough children from other nurseries so they have a "mix".
So pleased the stress of choosing a pre prep is over!