Hi David,
Agreed the ASC aka. “the hisser” is a MUCH simpler device. I have only ever had one, it’s on the 1948 MKIV engine that’s inside my 1950 MKV DHC.
Mine is so early it does not even have a blow-back valve, which is probably why a PO has put a small switch (looks period correct) below the streering wheel for powering it.
Once it’s off, it’s off as the spring closes the whole passage the solenoid opens when powered. The reason the switch makes sense to have on a very old car / engine is that in the original design, when you have ignition on, that wire going to the small solenoid (with bakelite covered small thumb nuts) will always have 12 volts (in my case -12V as the car is positive ground/earth) and I agree with Ed Nantes that it’s not always a great idea to have a live wire right next to the carbies.
The AED is great when it works perfectly, easy and always starts without any hesitation, BUT once it starts to misbehave it can really drive one nuts! 
So I thought I can live with that switch, and I used an amber one with a light as I thought it would also catch the eyes of any lady drivers reminding them that something is on and the amber light is a little bit alarming, but the car will not start cold without it.
Cheers!
PS. Here: