Magic!!
...& just loop through the karabiner & wrap round the tree & tie with a bow (or whatever) as normal?
Thanks
Well seeing as you upgraded to biners it seems a pity to keep tying it off at the tree. You can tye off and adjust your suspension from the biners now rather than at the tree. Lots of choices and personally I use descender rings as shown in this vid:
http://www.youtube.com/watch?v=_lOT7NkI-gU
Heres mine at the moment (tho I switched out the green loops with the black webbing you get with the DD as its' a bit thicker. )
Basicly the rings at like a the buckles on your rucksack work. I upgraded to 1" webbing but you should be able to do the same with the standard webbing as the rings work fine with normal cordage. Not sure how you could attach it round the tree, if you have two more biners you could do as I do, or you could just knot fixed loops into the end and thread the length through.
I like it 'cos its adjustable, much better than thaving to tie it and retie it. If you're dead set and want to just keep tying it off at the tree with a knot I suggest a marlen hitch as it's just a dead simple and strong knot.