Hi Mum to 2.
Don't actually have experience of 12 plus or Calday, but found this from the school admissions code (via Wirral learning grid)...
3a. Admission during Years 7 - 11
Pupils will be admitted to other age groups provided they have reached the required standard and that their admission does not prejudice the provision of efficient education and the efficient use of resources. The procedure will be administered within the school and the Local Authority informed of the Governorsâ€™ decision. Parents should contact the Headteacher who will discuss the arrangements for assessment of pupils.
Pupils may be re-assessed for entry into Calday at the request of a parent if more than a year has passed since any previous test. If it is determined that the boy is appropriately placed at Calday he may be transferred when places are available. Such a transfer will normally be made at the end of an academic year. However, the Governors will consider entry at other times during the year.
Hope it helps give you a starting point, can't hurt to give them a call...