The buyer has 30 days from the latest estimated delivery date which you should be able to see in your 'orders'.

If they have opened a payment dispute rather than an Ebay case, they do have longer to do that and you would need to have tracking proving delivery.

I would advise keeping your proofs of posting for a longer period - I keep mine for a year.

Your buyer has 30 days from the item's last estimated date for delivery to open a case for item not received.

Only a tracking number showing delivery will close the case in your favour.

If this is not available you will need to fully refund within 3 days so as not to receive a damaging defect on your account.

Just to ask was this by chance a GSP item?  Then, all that's need here,  is tracking to show delivery to the GSP location in the UK.  If it was a GSP item and tracking does show this and a case is opened,  make it clear in the case tracking Has shown delivery to the UK hub.

@hanveneklaasslot-0 

Same as @jckl1957 I also file my proof of posting slips away for a year before disposing of them. 

Buyers have 120 days with most debit/credit card providers to issue a not received chargeback, and your only protection against that is a tracking number showing delivery. 

Were the tracking details not automatically uploaded or uploaded by you to the original order? If so, they should show delivery/non delivery.