Frustrating as heck this, we had to get the AA out earlier this year.

One thing you should try (especially if you've been bashing the back end of the key in frustration), is to insert the key a bit less than completely into the lock barrel. I guess there's some sort of depth-stop at the end of the barrel that can get bent, allowing the key to go in too far.
Once the AA man had got me 'in' such that I could get the barrel off, it became clear that the key was going in about 2-3mm too far, and so the tumblers of the lock were no longer lining up correctly with the cuts in the key (if pushed in all the way).
The tumblers, and the inside of the barrel, will probably need a bit of fettling ( see
http://www.omegaowners.com/forum/YaBB.pl?num=1213353737) if you do manage to get it off. Replacement is probably a better option if funds allow. I was too tight for that so a small tyrap around the hilt of the key has been an ongoing solution to the 'key goes in too far' problem. Fettling of the tumblers isn't too tricky.

If you do try to refurb the barrel, take your time, and write down which 'number' tumbler (they're all stamped) goes where down each side of the thing if you take them right out. Note: if you pull the key out with the barrel out of the lock the tumblers are free to fall out!
Best of luck,
Pete