At a loss, I finally consulted the available Hints, but it just says I'm to approach the dragon from the orb zone (which I did), drop the vial, then kill the dragon. This accomplishes nothing; the dragon still happily blocks the E exit. I then did what I almost never do: check a walkthrough in desperation. The only one available seems to be the venerable Mike Gerrard's one, but it just says (quoting):
The problem is: this cannot be right. I have already established that dropping the vial does not activate its property: you have to either throw or break it. Such a discrepancy makes me think the solution may be wrong on other counts, but after checkinbg other parts, it seems to be accurate otherwise (except for the endgame, which I obviously haven't tried). Killing the dragon from the west just makes it block the east exit, preventing us from progressing. Killing it from the east appears to do the opposite, but the east exit is still blocked. Putting the dragon to sleep seems to have no effect; after 1 or 2 turns it wakes up again, the exit is blocked regardless of the dragon's sleeping. Wearing the ring or not does not seem to solve anything either: not wearing it makes us visible and the dragon can then kill us, but the E exit is still blocked.When you encounter the dragon don't kill it from the direction of the warm pit as its body will then block the way into and out of the area where the orb is to be found. Instead you should kill it on your way out of the orb area by dropping the vial, to put it to sleep, and then killing it with whatever weapon you have with you.
Possibilities I'm considering:
(1) The walkthrough is wrong (actually, it is with respect to dropping the vial, but this is a minor point).
(2) The walkthrough is right for other versions of the game, but the Spectrum version requires a different solution.
(3) The Spectrum version is buggy, and the dragon should block the W exit instead when killed in the expected way.
(4) I have overlooked something, obvious or not.
Naturally, I find (1) and (2) improbable. Option (3) I cannot discount, because I have already found several bugs, but they don't affect playability. One is with the oldest SCR Adventures release which seems to use the same parser as the Foilkade Adventure 200: it has an uncontrolled database read in one of the checkerboard rooms, but it does not corrupt the game's state. The newer CDS release (white text on blue background, different parser) does not have this bug, but the vial can be dropped and broken infinite times (triggering its effect) because the corresponding flag is not reset. (!) Not that this helps a bit, though.
However, I am forced to admit that option (4) is by far the most likely. Anyone can solve the mystery? I'd really like to finish the map and write a proper walkthrough. Credit will be given as appropriate.