Interestingly it seems to be working fine now. The last three or four times I've turned the amp on (with the standby button, not the main switch on the back) the chromecast has been there and ready to play.
Gremlins! Gremlins everywhere
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Brian RR 2160 - Mission 774 towers / ELAC B6.2 speakers Chromecast Audio or Roku Soundbridge M1001 streamers