My elderly neighbour was having problems with her Freeview reception last summer.
I checked all her connections and as she had a relatively new aerial and TV I started to look on the internet for possible explanations/fixes. I came across the site you have linked to and I also came across an article which explains
and how Tropospheric Ducting or the Inversion Effect effects signals. Unfortunately on checking the forecast for our area it did not seem to be the reason for her problems.
I then found another possible explanation which can also cause similar Freeview interference. 4G. Honest it's legitimate and not just aluminium hat territory.
The site
at800 was set up under Government direction ensuring viewers who rely on Freeview for TV continue to receive it when 4G services launch across the country.
We had had a 4G mast erected locally over a year previously and she not had any problems originally but I contacted at800 and they sent her a free filter which I installed and her reception is has been perfect ever since. No more pixellation or sound interference.