I have recently dealt with your problem. Unluckily, when I came near the problem, which is situated in Tuatha's Garden aka the Pond Tomb aka Little Tomb in the Woods and in France, my vacation time was up. Also, unlike in one other place with the same problem in China, which was in Pangu's Haven I think, you cannot use zeneporting, since the walls are too close by. I read in another forum that you might try using a Ghost Potion. It allows you to walk through walls and float like a ghost for a certain period of time. I think something like this also happens in the Copper Mines in Egybt but I haven't been there.
Btw, I think you can edit it with testingcheatsenabled true, but I'm not quite sure. Anyways, good luck.
Edit: Just so you know, Ghost potion is made with the Logic Chemistry set, not alchemy.
