2015 update: quite a number of adders have been seen around Curbar this year, some of which have behaved quite aggressively towards people. They are often very well camoflaged so it's worth keeping an eye out when moving into new areas, picking up kit etc as a bite, although not life threatening to most healthy adults can still be nasty.
Open access land, designated under the Countryside & Rights of Way Act (2000) give area access rather than linear access as provided by public rights of way. It also gives a legal right of access specifically for climbing, as well as walking and other quiet recreation on foot.
Please bear in mind however that the landowner still has the right to restrict access for up to 28 days per year (often used on public safety grounds for shooting in moorland areas), and can also apply for longer term restrictions with Natural England (such as bans on dogs, or regular restrictions during particular times of year). It is important to check for these restrictions regularly as they can be added at short notice – all details for open access land in England can be found on Natural England’s website.
All group leaders should familiarise themselves with BMC guidelines for group use, see here, http://www.thebmc.co.uk/Download.aspx?id=350