Hi Steve,
What are you using to power the Pi? if you're not using a good quality power supply, e.g, the official Pi one it's not surprising that you're having issues. I had a similar issue and it was down to the cheap power supply not being able to power the Pi (in my case Pi 3) and TV HAT fully.
If you still stuck, I'm more than happy to help via teamviewer/remote desktop - just ping me on IRC. I'm mpmc in #hts on Libera.chat. Available throughout the day UK time.
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Hiro, codecs, unless he's actually using the Pi for playback and transcoding won't matter here, the DVB-T here still uses MPEG2 and T2 uses h264