The principle of your issue will be the same regardless, it's just sometimes useful to know which box you have.
You've not mentioned any issues with live TV, so I'll assume that's good. That's your TV signals proven ok. You've not mentioned OnDemand, that would be an internet issue.
If you're having problems with any recorded or paused/delayed programming, that points at a hard drive issue. A good test is to watch a channel live and see if it's ok (if not, that's probably a signal issue) - now press pause on your remote, and/or rewind, and play the programme. It's now playing back from your hard drive.
If that proves the diagnosis, then HDD issues aren't fixable and are likely to get worse over time. Calling VM faults should be the way to do - 150 (from a VM phone) or 0345 4541111 (from any other phone) and follow the "I have a fault with my TV service" options.
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