Like some others here I have only vague memories of this series, but remember being freaked out by it at around the age of ten. There seem to have been quite a few series in the 1970s that tapped a vein of supernatural terror, often with Celtic overtones or suggestions of folkloric nastiness. Another I remember vividly was 'Children of the Stones'.
The episode of "Shadows" I recall featured some people who were trapped in the bottom story of a country house or possibly a windmill converted into a house. They were being threatened by the trees outside - branches were bursting through the windows and poking around to get at them.
Would be interesting to see again, but don't know if it'd stand up now - such shows seem more intense when you're younger and less cheesy.