If you buy from Ebay or get them direct from USA, not only do you not have a warranty, you dont even have a legal piece of kit. It is not CE marked.
Only SRS-600CE backpacks are CE marked, and whilst these may be slightly more expensive to buy (due to the additional cost of the CE marking), maybe the cost of using illegal kit may be a lot higher if anything goes wrong on your customers property.
If in doubt check with Williamson Pumps (0845 226 3638) - they are the official UK distributor.