Another thought is the Modulator - would that have any bearing on it? Although the Sky+HD box uses the same modulator and WSS's no problem.

Recap:

Solo2 > Composite out (yellow cable) > Modulator > Coax Cable > TV Tuner (No WSS)

Sky+HD > Composite out (yellow cable) > Modulator > Coax Cable > TV Tuner (WSS works fine)

The Modulator in use is a 4 channel modulator so it's the same unit in both examples - I have even swapped the cables between the boxes but the same outcome occurs - So I fail to see how it can be the modulator, but I'm happy for someone to prove me wrong