Many thanks for the suggestions guys, I am continuing to look into all this stuff. I will follow up Kobo in particular, already looked at it but not registered yet. A couple of things:
I tried ebooks.com . I was very suspicious ( partly because of the name, also I saw a few negative comments floating around), but was pleasantly surprised to see that it worked OK. They had a delay of about half an hour between placing the order and sending me an email with the download link, but that's fine.
Doubtless places like ebooks and Waterstones have inferior prices but I'll take what I can get. The biggest drawback of course is small selection, but I think there's just no way round that.
Mostly I like to read non-fiction (topics like science, economics, maths, politics, philosophy etc.). A good example, a few days ago I bought a paper copy of "Factory Girls" written by a WSJ journalist about life in the factories of Dongguan. This is the kind of stuff I read all the time, and would happily buy 5 or 6 a month if I had access to them.
Although I read novels, it tends to be when I'm on holiday a few times a year. I don't think those sites like "smashwords" can really help me with the former (non-fiction, up to date) type of book, or am I wrong?
By the way I am finding the N618 quite a nifty machine
.. although I don't have much to compare it with!