Rear derailleur cable
Shifting had been getting slightly lazy for a few weeks, the kind of thing you ignore until a hill makes the chain jump two gears at once. The cause was an old inner cable with a bit of rust near the housing stop.
The job
Shift to the smallest cog so the cable is slack, undo the pinch bolt, pull the old cable out from the shifter end. Check the housing for kinks or cracks and replace any section that looks tired. Thread the new cable through, grease it lightly, and seat the ferrules properly in the stops.
Setting it up
Pull the cable snug and tighten the pinch bolt. Then use the barrel adjuster a quarter turn at a time while pedalling and shifting up, until it climbs to the next cog without hesitation. If it shifts down late, undo a little; if it shifts up late, do a little more.
Total cost was a few euros and about forty minutes, most of it spent looking for the 4 mm key I had put somewhere sensible.