Dont let Ford rip you off for anymore money. They really havnt got a clue. If you want to spend main dealer money then go to Vw Try contacting an Air Con specialist. There are lots of mobile ones about who will come to your house.
Its the alternator pulley, not the tensioner or the belt It has a clutch fitted to it and its wore out There is no need to change the whole alternator no matter whet the garage says, the pulley can be replaced Do a search on here, lots of info about it.
You say headlight bulb, im which case you dont want to be doing it throught the wheel arch flap, as far as i know this is just for the side light and indicator bulbs. I always change mine with the headlight in place. A bit tight but it can be done.