Nick, as Bill was alluding to, this issue with the 550 is well known, but it can come from a number of causes, and just about all of them are related to the different connections on the tank itself. Here are a few likely suspects:
1. Rollover valves on top of the tank (probably the most common)
2. Cracked fuel pump covers (2nd most common)
3. Hose from filler neck to fuel tank cracked or loose.
4. Crack in the tank itself (very unlikely, but it happened to one owner)
Almost all require the tank to be removed, although if you're a masochist and contortionist, you might be able to replace the rollover valves by just pulling the tank to the rear. You'll probably shed a little blood in the process too....
The most common method of diagnosing the problem is to pressurise the system with a smoke machine, and look for smoke coming out where it's not supposed to
Most modern garages have these smoke machines as they're frequently used to diagnose emissions systems issues.