OpenStreetMap
the current situation is that the boundary of the Somerset Levels and Moors is now relation 278627879.
At the time of writing this relation has the following ways as members: 207244544 207249332 54644027 207249333 207746811 207746818 207358221 207746813 190914419 29417895 207244546 153044009 153044010 72554425 148075256 207244538 207244543 Some of these were existing features (rivers) that I used as a boundary.
I also have (for the purposes of rendering) two ways tagged as natural=moor 208638587 - Western and Southern segment 210866783 - Middle segment
As a result of Potlatch misbehaving when trying to save an overlong way I also have the following ways created in error that probably ought to be deleted: 207882847 207881808 207881814 207882846
Discussion
Comment from EdLoach on 19 March 2013 at 08:39
If it is the Somerset Levels National Nature Reserve boundary you wanted, then the boundary will be available in their data downloads section which are licensed as OGL and so compatible with Openstreetmap’s ODbL licence. I’ve probably got a copy here somewhere (as at May 2012 when I downloaded loads of zips).