In answer to my own question - it looks like the problem might be the plane iron itself. It's a Lie Nielsen blade from a no.6 (about three years old) and I've had some difficulties with it in the past (edge chipping and folding over), but I've always put it down to my own incompetence. However, I picked it up this morning to plane some friendly Cherry and found the shavings coming out in a number of sharply defined ribbons. When I checked the cutting edge I could see at least five nicks (the last thing I had planed was some clean, planed Ash). So I resharpened, got rid of the nicks, and took a nice even, fine shaving. Sadly the second shaving came out in two strips and the cutting edge now has a fresh chip.
Joel