Repairing a motorcycle chin strap can be done in less than 10 minutes for £5 to £9 (in 2010). Requiring no special skill, virtually anyone can replace a chin strap.
Purchase a new strap suitable for your make, size and model of helmet; not all straps fit all helmets.
Locate the screw or button attaching the chinstrap to the helmet -- it will be at one side of the helmet.
Unscrew or unbutton the old chin strap and discard.
Button on screw the new strap where the old strap was buttoned or screwed. Tighten the screw, if applicable.
Write down your helmet manufacturer and model and bring this with you when shopping for a new strap to ensure that you obtain the correct strap. Determine if your helmet is under warranty -- in which case you may be able to receive a new chin strap or helmet free or at reduced cost.
Determine how chin strap was damaged. If the strap was broken during an accident check your manufacturer's manual to determine if the helmet should be replaced instead of repaired.