Viewranger is great. There are a few other apps that do os maps but none do it as well as Viewranger. Online mapping only isn’t a great idea in the hills but you can get away with it. The free maps work well on viewranger, too, and there are loads of extra features. I rarely pay for software but I was happy to pay for this. I’ve got IGN maps for Morzine, too. If you are considering it drop me a line before you register and I’ll send you a referral for a few free map credits, or if you know someone who has it already they can do it.
I don’t think 1:25000 is worthwhile. Its a bit expensive to get a reasonable area and you don’t really need it with GPS as you know exactly where you are without the need for loads of map features.