I did it at Osbaldeston last year, and in the 75cm class the final JO fences were 1mtr, and very stiff competition!!! There were 14 went through to the 3rd round!
Depends where you go, Barton was always quieter and not as big.
The 2nd rounds should be the same height as the 1st rounds. I went to the 2007 finals for 75cm and 85cm and they were no bigger or more technical then the qualifying rounds, the only thing slightly more complicated they had was a water tray (85cm) on the Saturday and a treble (75 & 85cm) on the Sunday
The 95 and 105cm's didn't go up as much in height as the 75 and 85s, well they didn't look to be much bigger than 1.10/115. However when i went in 2006 they were jumping in under flood lights!!