Following tutorials to create custom vehicles in UDK have so far proved fruitless and confusing, with many methods half-finished, badly explained or outdated. However, a little tinkering and late night browsing caused me to chance across an update in this thread:
Forum member XxXdanceXxX has made a basic 'Humvee' vehicle for the UDK based on the in-game Scorpion, and kindly submitted source files, found here:
This was a far simpler, stripped-down version of a vehicle plus its script, so it was easier to understand how vehicles function in the UDK. After an hour or two of following a tutorial from another fantastic source (http://code.google.com/p/steam-punk-pirates-mod/wiki/CustomWheeledVehiclesTutorial) I was able to get dance's Humvee working in-game.
Progress at last! And a big thank you to all those who helped on the forum along the way!