Red Light Permanently On

You need to check if the problem is in the removable cassette or in the system itself.
To see if the problem is in the cassette, remove it from its housing.
Then get a small magnet, and hold it against the small reed switch in the now empty housing (the one with the wires on it near the fuse), and have someone inside to see if the 'full' light comes on. If it does, then the problem is inside the cassette. It uses a small float, that has a permanent magnet attached. As mentioned above, you will need to see that the float lever is moving freely, and if necessary, replace the magnet.
 (The reed switch is comprised of metal contacts, that come together when a magnet comes near them)

 If this does not resolve the problem we advise that you contact a Thetford service centre to ensure you receive a correct diagnosis for your issue.