If you haven't been sent the collection code you'll need to contact eBay CS and tell them, it's possible that your seller didn't put the eCP code on the package. This code is input when the package arrives at the store and generates your collection code.
I’ve found that the easiest route to contact eBay Customer Services is via the link below where you can request a call back:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid