Don't worry Stan, it wasn't the flashing that concerned me it was the horribly distorted sound and the two went hand in hand. I wasn't really getting flashing without distortion. I seem to have got to the bottom of the problem now though. I use Windows Media Center as my interface for music, live TV, recorded TV and videos. It's really nice to use but unfortunately it uses Windows Media Player in the background for audio playback and that means I don't get bit perfect output. This is because Media Player doesn't natively support WASAPI playback. So I use a piece of software called ReClock which forces Media Player to use WASAPI. I've found this setup to work well up to now but it seems it doesn't like 192KHz. If I tell ReClock to use DirectSound output instead of WASAPI everything starts working properly. So I think I'll have to live without 192KHz for a while until I can figured out a way round the problem.
Thanks for the help.
Source: Apple TV 4K - DAC: Beresford Bushmaster Mk II - Preamp: CI AudioPLC-1 Mk II - Power Amps: Musical Fidelity 550K mono blocks - Speakers: Wharfedale Opus 3 - Cables: Mark Grant etc - Misc: Belkin PF30 mains filters.