I didn't get round to posting it (yet) because i was busy, but I read something about 67 mirrors remaining stuck and the following advice:
Originally posted by Silent Street The correct procedure (applying to the Pentax 6x7 and 67 cameras) is to depress the recessed button to the left of the lens mount, release the shutter once and then wind on (the wind-on lever should spring back to its home position when this reset is done). This properly resets the mirror and the shutter.
I did that with my KX and lo and behold, it worked! I still reckon the camera can do with a service, but this is good to know.
(I depressed the depth of field button with the mirror lock up switch, as that's what the KX has, then took a picture, then wound the film on and everything's back to normal.)