Sanitop toilet in my basement really smells ?

Hi I have a Sanitop toilet in my basement studio. It smells really bad. The cistern isn't blocked. I have put gentle toilet unblockers down it then used a heavy duty caustic soda to flush it through in case it is. It is fed by mains water. How can I check if the pump works properly? It does make the noise as if it is pumping the water up the pipes (and I can here water trickling through the pipes) but I'm not sure that it is pumping it all away. I can't really describe the smell but I guess it is similar to stagnent pond water? PLEASE HELP as I have to work really close to it!!!
Many thanks,
Dee