ok guys ive had my alternator replaced and the noise has gone! Im gutted that the idiot at the first garage who only changed the battery didnt tell me the alternator was on its way out, it would have saved me £300 changing the damn oil filter housing for no reason.....lesson learned. Now to the cost, ford garage quoted me £420 for my alternator to be replaced. I got it done at my local garage using ford parts for £300 so i'd recommend getting a few prices before going to a ford garage, how they can justify £80 plus VAT so £96 an hour is beyond me. thanks for all the help!