Seems that this was broken at some point. Part of the northern boundary was set along a road, someone later added a roundabout on this road and this resulted in the boundary becoming a full circle - or in other words, the entire national park became the centre of a roundabout somewhere near Chapel-en-le-Frith. The nominatum still points that this location. I tried fixing it but Potlach 2 just isn’t the right tool for tracking down the problems at this scale so I’m hoping that bringing to wider attention will find someone willing and able to tackle it.
Back in May, Blackadder imported the boundary from a Natural England OGL dataset to create:
http://www.openstreetmap.org/browse/relation/2176657/history