Solar chargers can be hit or miss. Especially if you're hiking around unable to expose it to sunlight (either it's in your pack or you're in the woods in deep shade, etc.)
That's true. You really need to live a lot closer to the equator than we do in the UK to make it realistic.
However there is a possible alternative. Your mobile one draws down its battery when turned on.
That's true.
Simply turn it off unless you need to make an emergency call and it will hold its charge for weeks (possibly months)
Sadly that isn't true any more.
It used to be true, but now we have smart phones and in general the, er, smarter they are the more they seem to drain the battery even when they're turned off.
I have a ten year old Nokia 8210 which has gone all over the world with me, and although it's showing signs of its age it uses no power when it's turned off. So the battery life can be as good as the self-discharge rate of the battery. Yes, batteries do discharge themselves, even when they're not in a piece of equipment. Some battery technologies are better than others in this respect but Li-ion are pretty good. My 8210 will let me make a call even if I haven't charged it for six months, which is often the case since I'm not a big mobile phone user. I also have a phone made by HTC which I think is the 'Legend'. I don't actually care very much what it is, because to all intents and purposes it's useless to me. One of the very worst features is that the battery is exhausted in about a week even if I never turn it on. When I complained about it at the place where I got it, all the guy had to offer was "the iPhones are just as bad". [RANT]Add to that the idiotic user interface, the mountain of completely irrelevant software in it which just gets in the way and the virtually unusable (attempt at a) touch screen keyboard and you have a recipe for putting the bl00dy thing back in its box and writing it off as a bad experience. Which is what I've done, and gone back to the 8210.[/RANT]
If you have a 'smart' 'phone and you want to have some emergency battery power, take a spare battery and don't plug it into the 'phone unless you need it. You might want to practice changing the battery in the dark, when it's snowing, with cold, wet fingers. I prefer a 'phone that just makes calls, and does it when I want it to (not often at all), without a lot of messing about.