As suspected, Tim condemned the rudder rib. The rudder spar holes had also been slightly enlarged by my inept drilling out of the rivets. However, Tim reckons this is salvageable.
Accordingly, I placed an order with Vans for one rudder rib. Actually I placed an oder for the "finish kit" and ordered the rib as a "by the way"...
The rib duly arrived a week later - $13, of which $8 was postage. How good is that?