Dolby digital set to always convert but no sound from centre speaker unless true 5.1. Suggests the conversion from stereo to Dolby surround isn't working. I can manually set the amp to surround but this shouldn't be necessary. Any suggestions other than changing the surround mode on the amp each time ?
I think you need Dolby Digital and "Follow Content" which, if my understanding is correct, will effectively bitstream the audio so that Dolby Digital 5.1 and 2-channel PCM will be passed through to the AV receiver/amp.
Dolby Digital does not mean that you always get 5.1 as some are broadcast in stereo, and some HD channels such as ITV HD and E4 HD are also broadcast in stereo. Incidentally I don't see why you'd want to convert stereo to surround sound as I think it's best to hear what is broadcast.
Dolby Digital has the capability of providing five channels but is not required to do so, indeed it be entirely wrong for DD to produce fake 5.1 from stereo input. There's no shortage of stereo from DD, look no further than BBC1 HD, BBC 2 HD and Sky Witness HD.