Can be known as the drive belt and sometimes the fan belt. Just ask a Ford dealer for an auxillary belt and give them your reg, they'll get the right one (hopefully).

The one on my car wasn't squeaking, was just cracked and old, but the one on my dad's defender used to squeel like fuck when you started it, I changed the belt and it's all good.

Look in the haynes manual under auxillary belt? I think that's what they call it in mine.

If aux belt change doesn't stop the noise, then it could be the timing belt tensioner on its way out, but I don't know if squeaking is a symptom of this.