I bought one from argos a couple of years ago as I needed one there and then, and its been great.
Not sure it can manage as much as 5 charges on an iphone though, probably only 2 or 3 I would think.
IIRC an Iphone5 battery is about 1,400Mah?
So you are going to need one that is about 7,000Mah at the same voltage, and in fact you will need to add a bit to that as they are of course not 100% efficient, so I would be looking for a 10,000Mah one to be really confident it could do what you want.