From the HES website:
Organisations that can help you find skilled tradespeople and professionals to work on traditional buildings.
You will need to use the services of skilled tradespeople and sometimes professionals to help you look after your property.
Trade bodies and professional bodies can help you to find members who are experienced in particular types of conservation work. Some trade federations can recommend members with skills in traditional building methods, particularly in areas such as roofing and leadwork.
Before selecting an individual or company’s services, view examples of previous work, ask for references and try to visit a recently completed project. This can help you to assess their knowledge, experience and quality of work. Cost is only one consideration.
For added reassurance, you may wish to use a professional who holds an accreditation in building conservation. The Royal Incorporation of Architects in Scotland (RIAS) and theRoyal Institution of Chartered Surveyors (RICS) hold details of suitably accredited architects and surveyors respectively.