Assuming you've rebooted your box once and the symptoms are persisting, you are correct to think that this is a service fault (causing poor TV signal) which VM need to rectify. The quickest way to do this is to call VM (150 or 0345 4541111) and report your issue.
As an interim check, see if any of the following channels are affected (depending on your package)
Syfy HD, Comedy Central HD, Alibi HD, TCM HD, 4-Seven HD. If any of these also have the same problems, it is very strong evidence of a signal fault.
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