This is Taleporia tubulosa, one of the Psychidae. The English vernacular for this family is the rather charmless 'bagworm'. Perhaps the Dutch 'zakdrager' is better. Tubulosa is one of the easier species to find as the case is large, 15-20mm long. It's triangular in cross-section and coated with fine particles except at the fore end where these are coarser. Tree trunks are a good place to look - cases can be found in any month of the year. Typical habitats in Somerset are wooded areas with little undergrowth. I've seen it in holm oak woodland above Bossington; on beech trees at Holford; and on planted field maples at Merryfield Airfield.