I've just purchased a Sony KDL-40D3550 TV with a Sony HT-SF1200 speaker system. I've connected the two together via a HDMI cable. When the TV sees the speaker system the picture goes black and I get no sound - is it possible to root the TV sound through the speaker system?

i dont think your ment to conect the 2 together via HDMI, the sound system will have a HDMI conection for things like blu-ray player or sky HD etc then pass the picture to your tv, if your tv has an optical or coaxial output use this.