Yes, some sort of straw byproduct Stig so readily available here. Google tells me: Thatched roofs were the primary roofing material in Britain for over 1,500 years. The Normans introduced thatching to the British Isles in the 11th century, and it became the standard roofing method. Thatched roofs were used on family homes, castles, churches, and university buildings.
As for longevity, I asked the guy doing one here in the village one time and he said this was last thatched 40 years ago! From staying under one on holiday as a child I can vouch for the waterproof qualities and also the fact that it's home to almost every species of spider in the UK!