I phoned Sling last night, just in case they could help. Unfortunately they didn't offer many suggestions, only asking if I'd tried a reset (held button in for 10secs) or that the power supply could be at fault.
If the power supplies ever come back in stock, I might try that. Although as the Catcher is powering up fine, I can't see what that would be the problem.
It seems to me that it's having a problem installing the firmware it is downloading. Can I get the firmware from somewhere else and install it ??