As an alternative perspective I carried the Garmin 62S across on the 2012 Challenge and it survived the deluge of the infamous Sunday admirably. I've had it for a couple of years and it's absolutely bomb proof - you can drop kick it across the car park and also submerge it and it still keeps going. I bought it on eBay as well as the OS map software and got a bargain but. I have to confess to not having used it to it's full technological potential and you can't beat looking at the paper map for the best perspective but it is a comfort to know that it's there in your rucksack/pocket when your cold, wet and wondering where on earth you are??? So it will most definitely be accompanying me again this time

I'll second the use of an iPhone with ViewRanger.
Although I carried paper maps I used it on last years Challenge mainly for checking my position, although it came to the fore whilst following a bearing in the fog to the Shielin of Mark bothy.
If you have any type of reasonable smartphone there are several free apps which will just give you your grid position.
I carried my phone in a drop-proof case inside a ziplock bag, had no problems.
I also had a "Power Monkey" battery pack to charge the phone up whilst away from a powerpoint. I was able go for 4 or 5 days before needing to recharge the "Power Monkey"
If you haven't already got some type of smartphone it probably be cheaper to buy a cheap second hand one with a GPS on fleabay and load up the apps above.
But do carry paper maps!
A quick plug for Dash4it.co.uk, I ordered some Landranger maps late last Friday night, they were posted Saturday morning and arrived first post Monday. Cost £4.54 each and free postage.

Hi John J. Your friend is female - right? I found this GPS too large to fit comfortably in my (tiny)hand and therefore awkward to use, something I didn't realise when I ordered it on-line.
Luckily the unit happened to be faulty and I was able to return it almost immediately in good conscience.

Thanks Joy, and everyone else of course.

Your responses have given her food for thought.....she's now COMPLETELY confused!

On the basis of your replies I've downloaded Viewranger (free) on to my rather old HTC Smart(ish) phone. For the princely sum of £7.49 I also downloaded the 1:50000 OS maps that cover NW England - I'm having great fun playing with it.

JJ

I went for Viewranger (I purchased the whole of the uk in 1:50K (cost about £75)) on a smartphone. I chose my phone specifically for this and got one with replaceable batteries (a Samsung Galaxy S3). I also have a waterproof case for it.

I navigate from 1:25k printouts (from grough route). On the challenge I carry one paper copy of my route and fwa at both 1:25k and 1:50k and the phone is for backup (my map carrier might blow away or get left lying on a hill side), a fwa off a fwa (etc) or when the track you are on in a forest doesn't go where it is marked on the map and you need to know where you are.

The phone will go through a battery if I spend all day using it for navigation but it will last 3-4 days in flight mode using it for audio books in the evening and the occasional phone call. So with 2 spare batteries I feel reasonably confident.

But also I use it for navigation in the car (it makes finding your way cross country so much easier when you have OS mapping of everywhere) and it has become my only method of navigation when I go out cycling (yes, it will let me down one day and I will end up using road signs, but it hasn't happened yet).

Colin.