mummyjee, I don't know how boys' 'collective' SET differs from past school-specific exams for girls, but here is what we found useful preparing for it.
In terms of books:
CGP 11+ series - work books with assessment papers, maths and English;
11+ First Past The Post - cloze procedure and maths books;
Schofield & Sims Mental Maths book 5 - very useful; run out of time to go through book 6;
GL, Letts and Bond multiple choice English comprehension and maths practice test papers.
We did a couple of Bond maths assessment books early on in year 5, but found them not challenging enough, so switched to CGP, which my DS found much harder. Bond English comprehension books are good, but not relevant for SET, because they are not multiple choice.
In terms of practice:
From about Easter, we focused on making sure that DS completed the papers within the allocated time, and ideally with 5 minutes to spare. Both maths and English SET papers had a lot of questions to go through and it worked out on average somewhere between 30-40 seconds a question. During the last week before the SET, we practised speed-shading of sample multiple choice answer sheets - I made my own based on an example shown on Sutton Grammar website. Children often run out of time because they spend too much time filling in the answer boxes.
Make sure you sign up your DS for Sutton Grammar mock tests in summer term - places go quickly, so keep an eye on their website from January. The mocks definitely helped my DS's confidence.