interior light stays on
 

Am I right to think the switch that turns the interior light on/off is part of the door lock actuator? But is it internal to it, or external?

I'm wondering if a squirt of WD40 in the right place would sort it, or is it inaccessible?

You are right, there are two micro switches in the lock assembly, means taking the lock out of the door.
Some people have has success by packing the switch with a bit of tape...
Several posts on here about it