The other day I had to troubleshoot a DVB-T reception problem. All three tuners in my HTPC were showing packet errors every few seconds (the picture/audio were of course breaking up too). I eliminated various connectors, splitter, adapters and turned off everything that could be possibly causing causing interference.

I stripped back all components to the point where all I had was the computer with one tuner connected on a single run from the masthead amp and still the problem persisted. I was resigned to the fact that I'd have to get up on the roof and check out the masthead amp and aerial so I put everything back together for the day.

The next day in the middle of the day I tried the TV again. Guess what - no packet loss. Everything was perfect on all three tuners. When I got home in the evening about 1600 everything was running fine until about 1715 when the packet loss started again. Something was causing interference. Given that I have very clean runs and all quality quad shield cable I figured it was most likely coming in from the aerial.  Given the timing, I formed the theory that it could be to do with the street lights. I confirmed this when the problem went away the instant the lights went off in the morning.

Initially I thought it could be the new LED street lamp that had just been installed but it tuns out it was the lamp on the corner of the street - the LPS bulb had gone faulty and was trying to start up very few seconds and this was causing very bad noise. The council came and replaced the lamp (with LED) yesterday and now everything is fine.  

I had also been experiencing issues with my USB over CAT6 (~20M). The USB kept disconnecting and re-connecting. I suspect that was the same cause but that's still to be confirmed as it was more intermittent but it hasn't happened again so far. 

Hopefully this post will help someone else with an elusive Freeview HD issue.