My wife has a 2003 1.4 Ghia Fiesta which I very rarely drive. I noticed the other day than when I make a left turn the handbrake light illuminates then goes off once I'm straight again. Does this mean I need to top up the brake fluid? 

Your handbrake is separate from the hyrdrualic brakes, so no you shouldn't need to top up (worth checking the level anyway). My guess is the handbrake switch is faulty, either due to a bad spring inside, dirt wedged in, or a bad connection somewhere.

From a problem solving pov that's where I'd start, it's the most likely thing to go wrong. I can't thing of what else would be on the circuit. It's just switch gets pressed, light's up dash, plays sound if you try to drive (done through the ecu I'd have thought). 

It's worth checking the fluid level, although the light vines on and off with the handbrake it's also a warning lamp for the brake system.

I agree with MarksST, at least I know that is the case with some cars, but I don't know for every model.

When checking brake fluid, you need to be careful to check properly. On dual circuit brakes the reservoir can have a kind of divider in it which goes part way up. this means that if one circuit gets a leak then it does not drain the fluid from the part of the reservoir that provides fluid to the other circuit.

So don't be mistaken by the level being up on one part and not the other. (a friend of mine crashed because of this- brake fluid level light on, he checked, thought OK and thought light must be faulty, but he did not notice one half of reservior empty. The car was written off, he was OK, one of the two circuits still worked, when one of the two circuits fails the car still has some braking but it is much reduced and a bit scary - but much better than no brakes at all- prior to mid 1970s roughly, single circuit brakes were common)

If the fluid is low, where has it gone?  check the whole system for leaks. rear wheel cylinders could be leaking inside the drums. I have had many cars that have done as they get old. if your rear wheel cylinders have never been replaced then a 2003 car could well be of an age where they are going (replacements are cheap though). if they have been leaking a while you may be able to see damp areas without taking the drum off. and may notice the handbrake performance has got worse from brake fluid on the brake shoes in the read drums.

As brakes wear (particularly front brake pads) the fluid level does go down a bit as more fluid is down in the calipers (and then gets pushed back up into the reservior when the pads are renewed) but that is not normally enough in my experience to make the light come on, but it may do so in some cars I guess.

4 hours ago, isetta said:

As brakes wear (particularly front brake pads) the fluid level does go down a bit as more fluid is down in the calipers (and then gets pushed back up into the reservior when the pads are renewed) but that is not normally enough in my experience to make the light come on, but it may do so in some cars I guess.

Fully agree; all brake fluid reservoirs hold enough fluid to accommodate full depletion of the friction material and then a bit more capacity on top of that. To design it any other way would be dangerous as you'd run out of fluid within the service life of the pads (and discs/drums). Consequently, aside from bleeding the system, there should never be a reason to 'top up' the reservoir and doing so can hide the fact you might have a leak, particularly a small one where the other telltale signs might be easy to miss.