We are looking for cheap but decent accomm on the islands - any recommendations? Also which islands recommended from above - electricity on all yet?

Generally speaking Don Khong is a bit quieter than Don Det and Don Khon. I forget what I paid for accomodation on Don Khong. It wasn't expensive and the room was fine.
WeiLong in Laos
80-100,000 you can get good rooms with electricity all the time on Don Khong, close to where the boats land...

$6 to $10???
Hmm - my bungalow on Don Det was $1 - ok, without electricity and a bit off the main scene... But with a wonderful farmer, on whose corn-field right on the river bank the two bungalows were placed, who brought me fresh fruit several times a day, came along just to chat - without a word of English, of course, and who often envited me to dinner without charging me anything... I did paint some boards, so more people could find this place - stayed there two weeks, my absolute top-hero in all of the 9 months, I travelled in the area: Mr. Boun Song!
If you keep on walking on the sunset-side, about 200m behind the last bungalow-place/restaurant, in the very next small peasant-village, the two (by now, it's three) huts are standing right next to the path, facing the sunset over the Mekong, no generator, plenty of animals, the kindest of people, an oillamp in the evening, hammock on the veranda - it's paradise!
It really is a shame that you seem to need electricity...