Update of sorts.....earlier this evening the picture completely broke down and the audio was working fine. It looked like when your digital TV reception is playing up, but plugging the STB directly into the TV proved that TiVo was the source of this, not the STB.

I'm not sure whether that makes it any easier to diagnose. I'll run diagnostics on the drives when I get a chance in the next few days and probably do a fresh image.