My heater fan makes a NOISE - AS IF A LEAF OR SOMETHING'S GOT STUCK IN THE FAN. How do I get at it to see whether that's what has really happened (it could also, I suppose, be very badly worn brushes). Do I have to take a lot of bits apart?

I've read in other posts that the heater fan assembly is dead easy to remove, just three bolts, behind the glove box.

After discovering leaves and crap clogging up the air intake water drains in the vents just below the wiper arms (which was causing water to not drain, pool up in heavy rain, and leak into the passenger footwell), it wouldn't surprise me if some debris or leaves got sucked into this and into your heater fan. Just a thought.

3 bolts yes but you also have few bits of trim, brackets and some wiring to move.