>Many monuments are multi-phase. Which phase do you restore to?<

Agreed, deciding on the phase is not easy. You also have to make a clear distinction between 'restoration' and 'conservation'. Restorers might try to restore something to what they think an object or structure once looked like (eg sticking new arms on the Venus de Milo) when it's obvious that that can be open to awful misinterpretation. Some mad restorer might think she's scratching her bum for example (the Venus de Milo that is not the restorer :-). Conservators would tend to say, "Look, the arms are gone, we'll probably never know what position they were in, but we <i>can</i> save the rest of the statue from falling apart."

The difference between restoration and conservation is that the former works from a premise that we know what the original looked like (pretty well) while the latter is really only concerned with preventing what remains of the original from further degradation.

The difference between conservation and restoration gets kinda interesting when, for example, buried stones are discovered at Beckhampton; presumably the stones there are more or less in their 'original' position so what's stopping them from being re-erected? Aesthetic considerations? Maybe. Financial/agricultural/political considerations? Probably.

The, "Which phase do you restore to?" is the easy bit - the financial/agricultural/political considerations are the harder ones to resolve.