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