Thetford cassette overspill

I wonder if anyone out there can shed some light for me.  Every time I fill the header-tank on my Thetford C250, an overspill of about 1pint appears in the toilet bowl. I've taken the top off the tank to find the pump sitting on the bottom because the bracket that holds it to the side of the tank is broken, but that doesn't appear to be the cause of this problem. The only explanation I can think of is that the 12 volt cables may possibly be under water when the tank is full to the top and a possible short-circuit occurs which runs the pump until the water falls below the level of the cables. Feasible? Anyone had this problem? Any suggestions gratefully received!