Amalgamated Dental is now part of the international company Plandent.
As far as I recall the last decent dental unit badged as an "Ash" (derived from Claudius Ash, one of the "legacy" companies that formed A.D.) was the Ranger - which was a traditional dental unit (as those lucky enough to attend the Arborfield course back in the 1970's may remember with some fondness).
After that they tried their hand at a cheap "mobile" unit (corrugated umbilical tubing being much in favour), similar to the Tridac units (also cheap) supplied later on to the MOD. This (the Ash unit) was indeed the Sirobox.
Apparently Siro can mean "slim", or "graceful" (which may have been what A.D. had in mind); but Siro was also Virgil's teacher. Not to be confused with Sirona of course; which I think may mean "goddess".
There is a nice photo on line of an Ash Sirobox (still installed) in an abandoned prison in Northern Ireland.
Value of the unit in the attic? Approximately zero, I would suggest. Although i believe that the Science Museum in Kensington still holds some historic A.D. items.
However, it must be said that the photo shows the unit to be pretty much complete, so maybe someone out there in the world may be happy to have it donated, even if only as a source of spare parts.