Your DVD and/or amp are producing noise on the powerline which is drowning out the X10 signals. You'll need plug-in noise filters between your DVD & amp and the wall outlet to fliter out the noise. I put all my A/V equipment on power strips with those strips being plugged into filters.
Look at the AF100, AF120, AC100, & XPF at
[Link: homeautomationnet.com], that's what we use in the US, then look for the comparable devices on the UK X10 sites.