I don't know why people are so scared of main dealers? Their not highway robbers you know. Sure they can be expensive but only if you just say yes to first quotations etc.. They will negotiate just like any other buisness!
If I had a problem like yours with my brakes I wouldn't have gone to the monkeys at halfords! Take it to Lexus, get them to diagnose the problem correctly, get a quote from them and then go off to find it cheaper at an independent! :)