Read the final paragraph on this page to get an idea of what's gone wrong (and why it's likely to be linked to excessive or harsh braking):
http://carparts.break...pares/Abs-sensor.aspx
Then use the part finder, at the top right of that page, to get cheap used parts. (I've used it plenty of times, without any problems. It's also the first place that many local garages will look for replacement parts). Then get a reliable independent garage to fit it for you. It will save you hundreds of pounds.
If you're still unsure, ask an independent garage for for their quote. They'll be able to advise you regarding the suitability of getting hold of a used part. Even if they have to go through a Ford main dealer to get a new part, they'll still charge you much less for labour. (I work for a Ford main dealer. Staff get a third off labour charges but it would still be far more expensive for me to get my car repaired at work than at my reliable local garage).
Chris