In reply to TobyA:
I've checked the t and c's on the Arcteryx website and also had an email back this afternoon.
"What is and what isn't covered by warranty"
Defects in materials or workmanship are covered for the practical lifetime of the product. Material refers to zippers, buckles and fabrics. Workmanship refers to seams, seam tape and construction. Damage due to wear and tears such as rips, tears, abrasion or UV degradation, misuse or neglect is not covered under our warranty policy but may be repairable for a fee.
As it was a hem cinch cord that just snapped, I'm pretty positive that the construction above covers it. The email from the chap stated he agrees with me and is unsure why the tailors shop stated that it wasn't covered under warranty but it may not be covered by the practical lifetime of the product and covertly suggested that over use of the cinch cord could make it deteriorate!
"What does practical lifetime of the product mean?"
"We build our products to last over extended periods of use, but nothing lasts indefinitely. Fabrics will deteriorate and fade over time and moving parts will wear out. We will cover your product under our warranty policy until such a time that we have deemed the product to be worn out beyond reasonable repair. If the product is showing the signs of its age—for instance, the fabric has become thin or faded, there are rips and tears, the zippers no longer catch, or the cuffs are fraying—we will take this into consideration when assessing your product for a warranty request. All gear will suffer from differing degrees of wear-and-tear, depending upon the user. Equipment used by an outdoor guide for 150 days a year will by comparison, degrade faster than gear used by a weekend skier. By taking care of your equipment you will ensure a longer lifetime for your gear. We recommend that you visit our Product Care page for more information on how to wash and care for your equipment. Use your judgment when assessing whether your product is likely to be covered under our warranty policy. It may simply be time to replace your product, and we hope that Arc'teryx will have the opportunity to provide you with superior quality products and service in the future"