I'm curious, how can you say that a "stress fracture" (which, to me, translates as "someone placing undue/unexpected weight/pressure suddenly on one part, which leads it to break") is "obviously... a manufacturing defect"? I'll happily agree that it may be, but I wonder what makes you so sure it "obviously" is?
Anyway, if you're certain that the cot/bed has only ever been used as per the instructions and not been subject to undue stress, (eg, an adult using it as a ladder) you have rights under the Sale of Goods Act. (Someone else may care to update this, it's too late at night for me to go googling!)
Essentially, the goods that you bought must be "fit for purpose" (and a cot-bed, especially one costing around £500, should last waaaaay more than 19 months, if properly used) and "of merchantable quality". As it is plainly not "fit for purpose" your first recourse is to the retailer, not the manufacturer. Contact the people who sold you the cot/bed, and tell them that you are rejecting the goods as "not fit for purpose". Your legal entitlement is to your money back; not repairs, or replacement, or vouchers - your actual dosh.
You can, of course, choose to accept an offer of repair/replacement/voucher if you wish; but be aware that if you do, you may no longer have the right to your money back.
If the retailer is difficult about giving you your money back, involve your local Trading Standards office. You have rights, here.