This (below) is the guilty part, photographed (none too clearly) on my phone. You can see the slot where the woodruff key went, and see how it's worn. I wanted to post a picture of the key too, but it's gone missing. It was very obviously deformed, with a number of grooves in either side.
The fuel pump itself is mounted on that plate on the right. It is gear, rather than belt or chain or cam, driven.
This (sadly out of focus), hopefully, is what it should look like. This is the spare entire assembly, complete with nice clean precise slot for the key.
And this is what it looked like after a visit to the local machine shop, cleaned up and with a new key in place.
There are more photos, mainly of the replacement parts, here, should you be so inclined.