In reply to Wonrek:
From what I've read the rucksack is not fit for purpose.
As a result I would seek a refund.
As previously stated the contract is with you and the retailer.
I would send the rucksack back to them via recorded delivery asking them for a refund.
they will either send it back to you or refund the money.
If you recieved the rucksack back inform them that they are in breach of contract and if you have not recieved the moneis within 7 days of the dat of the letter proceedings will be brought via the small claims court.
Keep all records, reciepts, etc.
I would use the rucksack in December if I were you and work around the faulty zip. as it will proberely take 3 months to get to the small claims court.
However you may find that when they get the paperwork and realise that you are going down the legal avenue you may get your money back.
https://www.moneyclaim.gov.uk/web/mcol/welcome
it's not to bad and when go to court it'll be them you and a judge sitting around a table.
It's not that daunting really
Richard