DYSON DC04 Brush Roller Replacement?

Hi,
my brush roller (at the bottom) disintegrated the other day and I need to replace it. I'v located spares but am not sure how to do it.

I can see that the roller pulls off, but I am concerned that once I take it off the belt will disapear into it and I won't be able to get the new one on.

So in short, what's the procedure to replace the brush bar on a Dyson DCO4? (Hopefully I won't have to take it to bits too much)

Regards
Anthony