If it's the same problem on both boxes, then the obvious thing C/S will ask is that you've tried a box reset. It's a tick-box they have to go through, along with "are your cables all finger-tight".
Looking at the frequencies those channels are carried on, C5-HD shares a mux with Film-4 HD, S4C-HD, Sky History 2 HD, Sky Sports Action HD & Sky Sports F1 HD.
It might be worth checking some of those channels, just to reinforce that you have what sounds very much like a signals issue. VM need to physically intervene with those - either a cabling fault or box fault. If multiple boxes are affected, then it points at the common cabling.
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