O2 tested in 2009 --> 5 bar reception wandering around Allotment in fog, from near farm where you park for allotment, from just above Bar Pot, Kingsdale valley (though iffy near road further than Keld Head) etc. Ok reception in window of NPC (Mmm, thick walls). Managed to get a single text message in upstairs window of BPF (Dec 2008), usually have to walk a little way up the hill.
Massively improved reception in the last few years - perhaps they put another mast in that blasts up the valleys?
Mobile is mostly Line of Sight, so if you lack reception - head uphill!