Check your emails, even Spam, eBay messages and your order details in Purchase History, if it's not there you'll need to contact eBay Customer Services, they'll provide that essential code for you.
Be aware, you have 7 days to collect your item after you have been notified that it's in store.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
I do recommend contacting CS very first thing in the morning, when the lines first open, as there's more chance of Dublin answering.
@*biju*2010