5.1 Dolby decoder

I bought a dolby 5.1 decoder with 6 RCA outputs to go with a sky+ box. It was £26. The audio is fairly good - but its not automatically switching between 5.1 and 2.1 when the content changes. It only switches when the digital signal is removed and reapplied. Quite a few channels switch between 5.1 and 2.1 on the fly so the program can become garbled.

Does anybody know something that will work properly? I currently use the optical input.