You should first check to see if the tree is protected, as prior written permission may be required from or notification served on the Local Planning Authority or a licence may need to be obtained from the Forestry Commission.
You should consider appointing a reputable contractor who will carry out the works in accordance with British Standards Institute 3998:2010, 'Recommendations for Tree Work', and the current Arboriculture and Forestry Advisory Group best practice guides.
A tree may provide a habitat for plants and wildlife protected under the Wildlife and Countryside Act 1981 and other legislation. Trees with hollow crevices, for example, provide important natural roost sites for many bat species covered by the 1981 Act. Anyone proposing to carry out work on a tree which may be used as a roost site for bats or by other protected species should first consult:
Works to trees during the nesting season (generally March - August) may result in disturbance to nesting birds. All birds nests that are either in use or being built, eggs and chicks are protected under the 1981 Act.