I bought a shredder from Amazon about 18 months ago - £60 - it works OK but does not like continuous running - certainly no more than 5 minutes - it overheats and cuts out for 5 minutes - cools down and resets itself. It claims 11 sheet capacity but 2 or 3 seems to suit is best. I now no longer use it and resort to burning confidential papers.
If you have the paperwork you might consider taking it back to the store - do not tell them about what you did with the knife.
Good luck..., January 2015