How do I remove a Thetford C2/ C3 cassette toilet from the caravan?

My seal has worn on the cassette and as I was not on level ground "wee" was running over the cassette and into the base of the caravan
I have a toilet and folding wash bowl type
i have removed the screws around the folding bowl section and can see behind the toilet
I have unscrewed 7 screws from the front of the toilet in line with the contents indicator but it will not part
There are 4 screws in the base ware the cassette fits and will try these in the morning
Any ideas