If you have a child in year 5 and want to aim for a grammar school, start with the 11+ prep now. Don't wait until you are very sure, or if you don't know if you will move house, or what the syllabus is. I hosted a webinar on Tuesday to talk about the 11+, what you must be looking for when choosing a school, the syllabus to cover, timelines and the areas that you need to focus on. The parents' role in this prep is irreplaceable. If you want to go to a selective school, start the prep now and see where it takes you. Start small, make it work for you because you will learn as you go along- what schools are best for your child, how you will arrange transportation. Just like your child will learn the topics as they go along.
Don't just read it. Implement everything in it.