Dan - IF it was the friction plate letting go then there's a chance you'll have scored/gouged the inside of the box...

You really need to drop the box and have a look though pal - no real way of finding out the extend of the damage on the internet - as has been said earlier

I'm getting shit faced in London tonight - but if you're stuck - PM me and i'll call by on my way back tomorrow - see if i can help you