531 not defaulting to AC3 audio

I have no idea if its a configuration item or not, but my 531 installation isn't automatically picking up the AC3 audio as the preferred audio in transport stream type recordings. The source can be a .ts as produced by a PC DVB card or from a Topfield TF5000 PVR. Both MPEG and AC3 audio are present and so far its reliably picked MPEG over AC3.

AC3 is set as the preferred default in General Parameters.

Never mind... by sheer co-incidence, two completely separate broadcasters just happen to have AC3 audio problems at the same time. What's the odds?!
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