I got a B in A level chem, but it's been 4 years since I last balanced an chemistry equation, so I'm not sure either.
I think it looks right and it's going in the right direction, to fit the sodium in, how about this?
2 Na2S203 + 2 Cl2 --> S406(2-) + 4 NaCl
Don't know for sure though
really rusty at this.