Sorry for the delay replying to both who've offered advice (thank you!) I didn't get the emails sent due to having to reformat my hard drive! So, to answer the helpful questions...
The issue occurs on live TV. It tends not to happen on streaming or OnDemand.
Box is connected directly with an HDMI lead with nothing in between.
The sound cuts out but doesn't always come back until the box is unplugged and then re-booted. I've found that a quick forward or rewind often sorts it (but not always) or changing channel up and down (but not always).
I've tried the change from Dolby / PCM on both boxes before and this also doesn't bring the sound back unfortunately!
Interestingly this didn't happen once on an old Panasonic TV, or a slightly less 'old' Samsung TV. I have used the slightly less old Samsung to test the boxes when they cut out and that TV never has a sound issue. Only the two newer TVs.
Has been doing this today regularly when changing between two channels (Sky Sports, but could be any channel!)