Fighting the troll in the girls loo (Year 1)

Fighting the troll in the girls loo (Year 1)

Apparently (according to the walkthroughs!) this battle is supposed to be easy but I'm getting nowhere!
Ok, so I have rescued the girl hiding in the cubicle, between Harry/Ron/troll destroyed the whole room and collected loads of studs (including wrecking the loos and floating on the water spurting out of them to get the blue studs on the ceiling!).
I've killed one of the troll's hearts by getting the bit of rubble on the floor to hit him.
I'm stuck on the part where he throws a chunk of purple rubble at me - it's supposed to get a purple glow so I can lock on it and levitate it back at him.
I'm trying the advice from another walkthrough that says once he throws it, I dodge to the side but I'm getting no visible glow on it. The only time it does is when it's so close there's no chance as it immediately hits the floor and knocks me out.
What am I doing wrong? Every time I get to this part, I'm struggling with this bit for half an hour. I have gone back in and tried this area about 4 times now. I end up just exiting the game, losing everything I've accumulated.

Am I just not getting the sequence of buttons right?
I make sure my wand is set to purple to levitiate. I've found you can't hold X and try to follow the course of the boulder being thrown.

I only bought this game a few weeks back - I'm an adult who generally finds the Lego games just fun and relaxing but this is proving to be the opposite!

I asked advice in CeX where I bought it, the member of staff who had played it thought if the glow wasn't happening there might have been a glitch in the game so he replaced the disc. Still stuck - aaaagggggg!!!!

Any advice would be really appreciated! Thanks