I feel your pain. For an English saddle, I was using a Bates with a wide gullet on my gelding last Jan. Didn't ride him again in an English saddle until last week...the saddle was resting on his withers. Took a close look, installed a medium gullet, and now it fits great - but I would never have guessed that riding a horse 4 times/week would change his shape to a narrower gullet.
With the western saddle, I just add an extra Navajo blanket and he's fine.
Something I don't understand...
Why doesn't someone make an English saddle with external padding, instead of internal. It could use an external foam pad like the Wintec foam pads, along with a custom shaped wool blanket. By changing the foam pad, you could account for small changes, and an interchangeable gullet would handle the majority of the rest. You could even have the foam panel come gusseted and regular.
Oh well. There is probably a good reason.
Also - off-topic, but just saw a good summary of saddle fitting here: Saddle Fitting