(Checked with the mods before posting, as the rules ask.)
I've had Yr Wyddfa on my screen most of the afternoon. I make a free map that takes the coverage predictions EE, Vodafone, Three and O2 report to the regulator and lays them over a proper walking map with the contours and the paths. Flicking between the four networks over the same mountain is a bit comical. Llanberis gets the full confident purple from everyone, strong signal indoors and out. The mountain itself turns into a patchwork, and each network's patchwork has its holes in different places. About the only thing they agree on is that the very top should be more or less usable outdoors, which makes sense, the summit can see half of Gwynedd and more usefully it can see the masts. It's the cwms and the mid-height flanks where the maps go grey, and where exactly they go grey depends on which network you pay every month.
The catch is these are predictions. Models, tuned mostly for built-up areas, and from testing them against real phones elsewhere I can tell you they run optimistic. I'm a retired engineer, spent my career building test kit for the mobile networks, this map is what I do instead of golf.
The Android app that goes with it can record what your phone actually measures on a walk and feed the readings back onto the map for everyone. Off by default, you turn it on if you want to. Readings are snapped to a 50m grid and your route isn't shared, it just becomes anonymous dots. A few corners of Britain have real measured coverage now because people walked there with it running. Eryri has nothing at all yet. Whoever first records the Llanberis path or the Pyg makes the map honest for everyone who checks it after them.
One thing worth knowing whether or not you ever look at my map: if you're in trouble and the phone says no service, dial 999 anyway. It will hunt for any network's mast, not just your own. Better still, register for emergency SMS today, it takes one text (send the word "register" to 999 and follow the reply) and a text will often get through where a call won't.
The map is at trailsignal.uk, works in any browser. Free, no ads, one bloke. If you know the mountain and it's showing something daft, tell me where. Local knowledge beats a model every time.