Many thanks folks. I've come to 2 conclusions. French engineers have no consideration for mechanics (I'd only just recovered from the trauma of headlight bulb replacement last year), and Haynes Manuals are not as helpful as they might be ("remove the fuse box cover", hah!). Special thanks to bob (Jan 2009) with advice re screwdrivers and not being afraid to give the rear battery cover a good yank. Confidence is everything in these matters. Also to David Russell (Dec 2009), thanks for info re amperage, but on my 03 C3 "Exclusive" (hah again!), the front row of fuses went 10, 10, 15, 10, 40, of which the THIRD 10A from the left was the washer motor one.
Jim M, February 2010