They have no obligation IF you caused the damage, or IF it is caused by fair wear and tear. I wouldn’t expect a retention mechanism to wear out in 6 months unless it was being (ab)used every single day in a dirty/abrasive environment.
Because you have accepted the goods (which I think anyone would say you have when you use a helmet for several months) you don’t have an automatic right for a full refund. You are entitled to repair or replacement though (if it is faulty). They have a reasonable time to do so – and I don’t think anyone would suggest it is unreasonable that they send it back to the manufacturer to see if it is appropriate to “repair”. Obviously it depends how quick the manuf. is at resolving it.
Within the first 6 months burden of proof is on the retailer to show that the goods were not faulty, after that you have the burden of proof to show they were.