Hi Lara, depending on the length of your bars then new feet are all you need and they are very easy to fit. Halfords tend to be a bit pricey imo and sometimes not as knowledgeable as you might like.
I have used these people for all my Thule stuff:
www.roofracks.co.uk as they are good value and can also master all the locks for you, which is useful.
As for roof-mounted bike carriers the Thule 591 is a brilliant piece of kit and a normal saloon or estate car can take three easily, or four at a pinch. This place has them on special:
http://www.fordonlineparts.co.uk/produc ... fAodohDCog