does anybody know that it is guaranteed to get a seat in st clement danes school, if we move to wd3 postcode?
You can live anywhere in WD3 if your DS is in the top 20 children based on 11 plus test score (assuming distance is not needed as a tie breaker).
You can live anywhere in WD3 if your DS is in the top 20 children based on music test score (assuming distance is not needed as a tie breaker).
If you are looking to gain a distance place only it will be dependent on how many places are available to allocate once all higher priority allocations (cared for, SEN, siblings, children of staff, academic places, music places) have been made and how many other children live closer to the school.
Have you started preparing your DC for the 11 plus test as it will be the only route into the Boys Grammar as siblings and cross siblings take the majority of spaces. To get a distance place for this school you will need to be within 200 metres of the school which would mean that you would not quality if you are in WD3.