Are you sure a 'summer crammer' would be the best thing for your DC? Perhaps some children thrive when they have to deal with periods of intensive work under pressure but a lot of them just find it overwhelming and end up tired, stressed out and increasingly anxious about the outcome of the forthcoming exams. Anxiety is not the best state of mind for 11+.
DS had most of the summer off and then did a few practice tests at home at the very end of August just to get back into the swing of things; he didn't want to do any more revision after the start of the school year saying it was making him worry about how well he would perform on the day, so we let him be. He was rested and relaxed for his exams and did very well.
Your DC may be very different and enjoy the last minute pressure of a 'summer crammer'. All I'm saying is consider carefully whether it would suit your DC before you decide.
Good luck with it all.