Not sure of this but.....
The proposed admission arrangements for Slough 2011/12 can be found herehttp://www.slough.gov.uk/services/22243.aspx
If you bring up the PDF file then you will see that langley is looking to change their arrangements..... as part of this they list the application date as 'address' date.
I am not sure whether they make exceptions for moving in to the area.... you would have to ring the school.
It seems that they are trying to cut down the number of applications to test......maybe to stop people using it as a trial for a preferred school test.
Hope that is of some help