Hi Roger,
Thanks for the reply. I'm certainly no expert but my understanding was Dolby digital gives 5 channels
https://whatis.techtarget.com/definition/Dolby-Digital
The amp info for the input/output would also seem to support this as it shows 5 channels from the virgin box.
If I set the Virgin output to Follow Content and let the amp convert to Dolby surround I get sound from the centre speaker.
Maybe the conversion performed by the virgin box doesn't work in the same way.
Recently the sound quality seems impaired and I wondered if there'd been a recent firmware update that might be the cause.
Its not a major issue other than I have to manually select the surround mode on the amp depending on the source output.