If your child has a low score, or is just by the pass mark, then really consider if it is worth it.
I disagree with this. It would make absolute sense if the tests were fantastically accurate measures of a child's ability to cope at a particular school - but they are not. They are often a measure of how well-prepared a child has been,or how nervous they were on the day. The difference in ability between a child scraping a pass with, say, a score of 210, and a child with a 'solid' pass at, say, 225, is likely to have been a handful of questions, more or less, depending on their birthday.
I have seen mentioned on here that once appeal children are admitted to the school, there is no evidence whatsoever that they would struggle more than the next child, or that those who score very highly remain at the top of the tree throughout school. The 11+, as it exists in Gloucestershire in particular, is a very blunt instrument which tests how good a child is at VR on a particular day. It is not a gold standard test for IQ, or for suitability for a particular school.