Harry Potter game - HELP I'M STUCK!

[Nosha123]

I have Harry Potter Order of Phoenix for the PSP (though expect the game is same on other platforms too!)
Am quite a way through it and have got really stuck now.

I am outside in the clock tower courtyard and get the hidden Portable Swamp off the wall and put it into the middle of the courtyard. I have to stack things by the wall so I can climb onto the roof

There is a large boulder which I can put up against the wall - but still cant reach the ' portable swamp'

Please help this is driving me mad