Cheap(ish) SatNav

[#1]

It would depend if it's just to get to say 'Market Street' in town, or the local post office, then if you have an android phone, Google Maps work great for that, you can even download it so it works offline instead of using 3g.
Or as a premium alternative, the CoPilot app is brilliant, and you can even enter a geo code, so it leads you to within 5 feet of the intended target.
If you want to purchase a decent but cheap sat nav unit as a whole, then you can't beat a garmin.
The Garmin maps are much better than TomTom, and the unit generally gets you closer to the address, as is more functional.
People rave on about TomTom, but unless you fork out hundreds, you only get the basic maps, which aren't brilliant.
I am talking from experience as a courier.
I have used most makes of Satnav over the past 10 years, and I find in a mixed rural/urban setting the Garmin maps are the most accurate.
In a mainly rural setting, the TomTom is SLIGHTLY better at directing you in time for a lane change or turn, but the maps arent 100% accurate.
As an example, even on a newly updated tomtom, a new road built 6 years ago  in Okehampton isn't present, the same with the new bridge in Barnstaple, it tries to send you on a massive detour, yet on the Garmin maps both are present.
My van has built in SayNav, and I uploaded the CoPilop SatNav to it, and even in a vastly rural area, it can take me to the door of an isolated cottage or farm house.
http://copilotgps.com/uk/
As an occasional user, and due to the ease of use, I'd suggest:  http://www.amazon.co.uk/Garmin-Touchscreen-Satellite-Navigation-Ireland/dp/B00BST8U68/ref=sr_1_2?ie=...
Hope that helps.

[#2]

@shutter I think you hit the nail on the head! I can read a map upside down, sideways, etc but SWMBO struggles even the right way up. When we are out she doesn't pay much attention to the surroundings so she often berates me for going 'the long way round' when, in fact, I am using the same route home as always
I can look at a map, decide the best(ish) route and make mental notes of roads (A299, M2, etc). I can usually find my way back without resorting to the map. SWMBO just hasn't got this ability.
To be honest, I'm reluctant to spend much on a SatNav as I still do most of the driving or we're together. On the other hand, for SWMBO's peace of mind, the Garmin as suggested by Devonian seems a viable option although I am still trying to convince her to use her SIII. Yeah - I know, a losing battle

[#3]

I`m like you... I amaze my missus when we go on long distance drives, and I can recall sections of the road,.... yet I may not have been along that road for several years... I can picture the road ahead, and describe it to her half a mile before we get to it !...
And, Like your missus,... she hasn`t a clue where we are sometimes. even on local roads... we travel to Chichester at least twice a week, normally I take the "country road" rather than the dual carriageway A27... however, we can be halfway along the A27, when she will say  "where are we? do we normally go this way ? "