Please do not be put off by tales of only super clever children sitting the exam - in SW Herts you sit one exam for all the consortium schools, so plenty of people will rank, for example, one of the Watford Grammars / Parmiter's / Ricky first, but also fill up the other spaces on the CAF with other consortium schools.
My DD had no tutoring, just some last minute familiarisation at home from me, using the Bond books, which I discovered after finding this site (& too late!) were not considered "the right type"
We hadn't even considered Parmiter's until their open day in the October when my DD was in Year 6. Plenty of people seem to be focused on secondary transfer from long before that! But my DD got an academic place on allocation day - in the first year when the academic intake was reduced to 45.
You say your DC is in Yr 5 - is s/he currently
level 5 in the 3 core subjects / top set in everything? If so, I should think they were in with a good chance. So why not go for it??!