Try the latest sensation that is Keane. They are one of the most high rated 'mellow' bands out of the indie scene in UK rigth now and I can only say I agree whole heartedly.
They are a 3 piece band WITHOUT a lead guitarist. One plays the piano, one bass and drums. Most (if there were any) of the guitar sounds were embedded into it later on so it's quite a refreshing change.
Songs to recommend:-
Somewhere Only We Know
This Is The Last Time