Carrera is a make that is exclusive to Halfords, so if it is a manufacturing fault they will be the only ones to attain parts or possibly to refund the item. This is rare however, as all bikes are legally checked by the retailers for faults before sale (Known as a PDI) and they will have records to prove this (you may have recieved a pink slip when you purchased the bikes).
The best way to repair brakes is simply to apply Disc Break cleaner, but failing that the simplest answer is to replace them.
The number one cause of brake failure is misuse, which includes failing to maintain them.
Hydrolic disc systems require almost perpetual maintenance to function, such as bleedings and pressure adjustment, and regular discs require lubrication ideally after each use, but at a minimum one every three months.
However, if they are squeeking through an incorrect distance setting this can be fixed quickly enough by any cycle specialist that should cost you no more than about £5.
Chris Tyler
October 2010