Found the issue - DelayVirtualizationType registry key for App Volumes had not been set on the golden image, as per the last line of Install App Volumes Agent Subsequently App Volumes was not...
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Found the issue - DelayVirtualizationType registry key for App Volumes had not been set on the golden image, as per the last line of Install App Volumes Agent Subsequently App Volumes was not fully mounting the applications on startup which meant that Horizon couldn't find them. *face-palm*