The Party Wall Act applies to most work carried out to party walls. If it applies, it means that you will have to serve notice of the proposed works on your neighbour(s). If they do not consent to the work, you'll have to appoint a surveyor to prepare a party wall award. You can find a surveyor using the Royal Institution of Chartered Surveyors’ (RICs’) Find a Surveyor service.
However, if works to the party wall are so minor, that service of notice under the Act is not necessary (eg straightforward repairs, such as replastering, or cutting into the party wall to add or replace recessed electric wiring and sockets) you can use a simple Party wall agreement to record the work to be undertaken.