Yep your power levels on the downstream are well out, can you go around and make sure that all coaxial connections to all VM equipment are finger tight, then power off hub for 30 secs and then back on, if the power levels are still that low then an engineer will be needed to resolve the issue.
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Everything seems to have suddenly started working again without me doing anything (coincidence that it's returned to normal immediately after I posted my question) - re-checking the router status, seems that downstream power is back to normal (all channels between 1 and -1 dBmV).
I guess a local problem that's just been fixed, or possibly a loose connection outside somewhere?