If you are still having problems read this.
It sounds like you broke the actuation arm that closes the blade by not closing fully before removing cassette previously. Go to the Thetford website and look out the Ask thetford send them online message how you can remove the cassette as it sounds like the broken part is stopping you from removing the cassette. Here is Thetford contact.
For those is Australia www.thetford.com.au the access for pdf repair downloads may differ as I haven’t check if it changed like the Europe website.
www.thetford-europe.com
On home page click on Products
Click on read more (Toilets)
Click on read more (cassette toilets)
Click on read more (C200)
Click on Service
Click on pdf Repair instructions- sanitation
Save file to computer then on disc for reference when in caravan obviously with DVD/CD player.
Lunar Solar owner.
Bob
January 2012