2003 Peugeot 307

Sounds very like the same problem I had a few months ago. Had new brake discs and pads fitted at the local dealer and paid  400 for the work, A few days later when ever I pressed the brake pedal I could feel a bad knocking/banging sensation up the brake pedal and up my leg and the abs warning light would come on. Took the car back to the dealer 3 times to finally get it sorted, They put it down to the abs sensors needing cleaning and adjusting.