Thanks for all the suggestions. We're pretty sure it's not a stiff link, and spent most of our commute this morning fiddling with the adjuster, but we'll try doing that some more. I'll try anything twice before getting a new chainring set... It's a bit tricky to sort out because it needs to be ridden for at least 100 m or so to figure out if the adjustments have actually made a difference, as the skipping doesn't occur all the time. Nothing seems amiss if we just turn the pedals by hand with the wheel off the floor.