Not sure about the Garmin, because I use the Satmap system.
But normally you have to buy in OS mapping software for your device.
Mostly on an SD card.
Which I am sure you know already.

When I got the Satmap I had to find a balance between cost and where I was going to use it.
So I originally got a National Parks set which did most of the stuff I would evenr need.
Later on I got Full UK 1:50000, but depending upon the amount of detail,
I would have thought the Scotland maps are a bit cheaper.
Not saying this is the best place to buy them, but it does show a good selection and a price to work off.

GarminMaps

I use Memory Map, but try this link http://www.talkytoaster.info/ukmaps.htm

I have just upgraded my GPS to the Garmin GPSMap 62ST and bought the whole of the uk on 1:50000 for the GPS unit. for 199 pounds I then down loaded for free Garmin BaseCamp for my computer. When i plug my GPS into the computer and start GarminBAse camp i can view the whole 1:50000 mapping in the Basecamp software to plan and plot my routes and save them to my GPS.

If the eTrex 20, can take the Garmin maps then i think this might be a cost effective solution.

Maps = http://www.amazon.co.uk/Garmin-Discoverer-All-Great-Britain/dp/B003CP0MSK/ref=sr_1_4?ie=UTF8&qid=1346697503&sr=8-4

BTW - The Garmin Basecamp software is the best mapping software i have used, better than memorymap and RouteBuddy.

Thanks for your input, guys. We have just done a tough tour of the Karwendel in Austria and hoped to get a bit of practise with the GPS, but as is normal in the Alps, there was a signpost at every path junction so no need!