Sorry to hear you're having an issues with the subtitles. I've tried to replicate this in the office but we've only got the On Demand content at present. There are no subtitles on there. For the next episode, due to air on Thursday the 25th, it doesn't show as subtitles available.
It's great to see that you've had a hunt for any possible fixes. I'm sorry that you've not yet been able to resolve this issue.
Could you confirm whether this is happening on both your TV boxes or just the one?
I'd like you to try manually connecting to the network by following these steps: press Home on your remote > select Help & Settings > select Settings > select Network > select Connect to the Virgin Media Service now.