Thanks wmittens. I'm absolutely convinced though that it was never audible until the car went in to the garage for some work on the air con/fan. The technician who worked on my car says that's impossible and I've just never noticed it before (I've had the car since last August). My husband drove the car today and he noticed it too. Strange huh?
If the car was having some work on air con and the mechanic left it switched off when he'd finished, and you normally leave the fan at a medium to high speed, then it's perfectly plausible that you've never noticed the click, but did hear it with the fan switched off. Now you know it's there, it's surprising you hear it every time (especially as it has concerned you).