You should get an allocation letter from the LEA and an offer letter from the grammar school.
In 2006 the LEA allocation letter was dated 01 March. I also received an offer letter from Slough Grammar (which was part of the consortium at that time) also dated 1st March, the envelope also included an acceptance form to be completed and sent back asap. This i believe is to allow the schools to operate their waiting lists as efficiently as possible.
The consortium, historically, have sent out offer letters for your highest ranked school where you have achieved the 'first round cut off' score. This may be posted slightly earlier to ensure it reaches you by the allocation date........ it could also be late due to the systems, communication between LEA and school and the postman having an 'off' day