Inappropriate repairs can not only destroy historic fabric, they can also cause structural problems which may be expensive to rectify.
Choosing a good architect and builder is important: not only will they understand the correct way to repair historic buildings, but they will also be able to liase with the conservation officer and have an understanding of what is or isn't appropriate, helping to avoid costly delays. Some techniques used to repair modern buildings can damage historic structures, so it is important to find professionals and tradesmen who are skilled in traditional building practices.
The local authority might be able to recommend reputable practices and firms and you can also contact the Society for the Protection of Ancient Buildings SPAB which has an advice line and can also supply you with a list of firms (Contact Douglas Kent, technical secretary 020 7456 0916).
The Building Conservation Directory may also be useful. It lists professionals, tradesmen and suppliers. Visit www.buildingconservation.com to read it on the internet. Alternatively you can buy a copy of their annual directory from the publishers, Cathedral Communications Ltd. (Tel: 01747 871717).
If you intend to undertake the work yourself it's worth knowing that
English Heritage,
Cadw,
SPAB,
The Victorian Society,
The Georgian Group
and some local authorities produce very informative leaflets with advice on appropriate repairs and materials.
SPAB also runs very popular and informative courses for home owners and professionals which give a good, practical, introduction to the subject.