Thank you Frankly Frankie for your informative reply - I've found your posts most helpful when considering all this stuff.
So, day 2 with the Oregon, 41 miles today.
Firstly, the screen thing has improved as I've turned up the contrast a fair bit; I could view the map reasonably well in bright daylight. I turned the contrast down again when riding after dark as I have the backlight on all the time and it's dead easy to read.
Have done 4-5 hours on a new set of NiMh rechargeable batteries and the battery meter is still on 3/4 so that's looking good. I have turned off the compass as was suggested to help preserve battery life. I have a spare set of batteries with me all the time so that's no problem.
It appears I can't get the proper Garmin map to work but having loaded AndyGates' OSM I'm really happy with that anyway. I need to find a good quality OSM of Germany for my tour in May but that should be fairly easy; I've downloaded a sample cyclists' OSM Germany map and will have a look at it in due course.
I've fiddled with the data fields on the trip computer and stuff like that so it's set up more as I would like, although I'm a bit irritated that on the 'Default' setting it insists on showing elevation (which isn't much point for me) at the top; I'd rather have something more useful - like the Time Of Day. A minor niggle.
Routes/Tracks/Courses. Well I'm beginning to get an idea of these. Having used only an Edge 205 before, which has a course (easy to follow breadcrumb trail you download from Bikehike) or a track (more faffy and doesn't follow the road, not something I've used on the Edge) I found I was all at sea with the Oregon. I think I'm getting a bit closer to what's what now, which appears to be:
Tracks - this is where you've been, rather than where you're going. However you can use a track to follow (which works rather more like the course on the Edge 205m but without coursepoints). The Tracklog continually records where you've been as one long ride unless you break it up by saving it. This is what I was trying to get at earlier when I said I couldn't delete the history - what I mean is that if I go out for three rides over the day, then plug the Oregon into Ascent, it downloaded it as one ride; the Edge would have it as three separate rides, several of which have laps (when I turn the unit off if going into a shop, etc). This was fairly automatic on the Edge but isn't quite as easy on the Oregon.
Routes - this is where you plan a route by plopping waypoints on the map as you look at it (I think). I imported a route from Bikehike and it didn't work at all well - it's just got HUNDREDS of waypoints which makes it hard to read.
Courses - don't seem to exist.
Trip Computer - this starts automatically when you turn the Oregon on and you can't pause it. You can reset the Trip Computer but then you lose its data. On the Edge the data that you'd see on its equivalent of the Trip Computer was what was stored, and you could zero it and then download the data later. With the Oregon it seems the Trip Computer is just for your own interest and the Tracks are where all the real info is stored (I may be wrong with this).
I have read the manual and haven't learned anything particularly helpful.
What I'd still like to manage to do is plot a route on BikeHike which has a couple of coursepoints (i.e. cake stop, lunch stop) which I can then follow on the Oregon, with a field which is "distance to next coursepoint". This is how I used the Edge 205 in pre-planned group rides where we were all following a route and I liked that feature. However it seems that the Oregon is less about pre-planned rides and more about using the mapping to trailblaze, which makes sense but seems to have done without a feature I rather like.
My husband is a sailor so he may try out the Marine side of the Oregon when the weather gets better although we would have to invest in a Marine Map, unless there are OSM versions.
I still really like the unit, particularly its display. The interface with the computer is less easy than the Edge and I'm still struggling with my pre-planned routes but I shall test out a track/route tomorrow and see how that goes.