Thanks fellow craftsmen for your glowing comments. The travisher is really a most pleasurable way of shaping the seat. I did once try an adze, but struggled with it. I haven't timed the job, Tom but I suppose it took less than an hour for these small ones in ash.
biff_kpv - The travisher is a hand made one but not by me. A South Yorkshire chair maker, Tom Thackray makes them. See http://www.mikeruane.com/tomthackray/ . The sycamore carved out really well. I used that because there was a suitable piece lying around. Traditionally, elm would be used but of course, that's much tougher with its irregular grain.