I think that most schools including Nonsuch are reluctant to let kids take the exam in year 5 for a variety of reasons. And no, if they don't get in they don't let them have another go - this is from the website:
The Selective Eligibility Test is for children who will be in Year 6 of primary school at the date of the test (17 September 2014) – expected date of birth range 1 September 2003 to 31 August 2004. Children can only sit the Selective Eligibility Test once. A younger child cannot sit the test in earlier years as a practice. The participating schools will conduct year on year comparisons of the names of registered children.