Thanks for replying. However I am not using a sound bar. I am using the TV speakers via an HDMI cable with gold connecters. The dropout is a function of when the box was turned on and repeats exactly at hourly intervals. It looks very much like a software issue to me although I admit it doesn't appear to be common.
I asked the engineer that installed the mini box if he knew of the fault. Although he had not come across it himself, he said he had heard about this before from a colleague.
It may only appear to be a minor inconvenience, but it is consistent and damned annoying!
Despite my misgivings I have disabled Dolby as you suggested and much to my amazement the sound issue appears to have gone! I suspect so few customers feed the sound through a hi-fi system that it is an uncommon issue. It would be nice however if (at least for my benefit) Virgin could get it to work at some point!