The website doesn’t have a great layout but the information is interesting and details are given of surveys in which to participate. There’s even a section for kids.
Also worth looking at if you’re interested in British mammals is Mammals Trust UK. There’s loads of information here.
The Tracking Mammals Partnership is a collaborative initiative, involving 24 organisations with a variety of interests in UK mammals, which aims to improve the quality, quantity and dissemination of information on the status of mammal species in the UK.