Jon, I'm double-posting my reply, in case your blog doesn't like links and images in replies.A lot seems to hinge (or henge) on (a) how current-day people perceive the object in question, and (b) which profession has custody of it.
By way of a few examples:
1) The SS Great Britain in Bristol.
It might have been a complete wreck at one time, but it was perceived as a great piece of engineering, worth the effort of restoring to better condition. Marine architects and engineers had custody. It was restored.
See:
http://en.wikipedia.org/wiki/SS_Great_Britain2) The Antikythera Mechanism
Now recognised as a fully working mechanical computer built c. 200 BC. Archaeologists have custody, didn't know what it was, and got nowhere with it. But engineers have taken detailed X-rays of the mechanism using a technique called linear tomography. The original is just too far gone to restore, but working replicas have been made.
![Image](https://upload.wikimedia.org/wikipedia/commons/thumb/5/59/AntikytheraMechanismSchematic-Freeth12.png/800px-AntikytheraMechanismSchematic-Freeth12.png)
See:
http://www.world-mysteries.com/sar_4.htmand:
http://en.wikipedia.org/wiki/Antikythera_mechanism3) Silbury, Avebury and Stonehenge
Not generally recognised as purposeful engineering of any kind. Barely and grudgingly recognised as astro-science of any kind. English Heritage has custody. It's all a mystery to them.
I did once suggest to EH that Silbury should be restored to its original condition.
![Image](http://www.the-sanctuary.biz/images/history/silbury.gif)
They must have been struck dumb with shock (or thought I was taking the piss) because I never got a reply. If you do poke them hard enough, they will produce stock phrases like "Splendid isolation" and "ritual objects". Intellectual tumbleweed.