Further to my equatorial adventures I can confirm that they’ve also gotten it wrong in Greenwich. 00° W is not actually on the line in the Royal Observatory; it’s about 120m out.
Luckily, the real meridian is in the park rather than behind the observatory’s entry fee, right next to the hot dog van. So you can all enjoy longitude 00°00′00” at no cost!
Turns out it’s actually the GPS system that is wrong. Wikipedia explains that the first GPS systems extrapolated from their origin in the US midwest, and the accuracy was such that by the time you’d rounded the planet to Greenwich, you were a few hundred feet out. Pity, but I guess they’ll need to relocate the Royal Observatory because those satellites won’t change their tune in a hurry.
It’s also amusing that when many countries were arguing over who should own the one true prime meridian, England won and France abstained from the vote ;-)