Just updating this thread to summarise my details.
I sold an iPhone (in original packaging) on eBay for £700.
I purchased shipping with InPost via eBay Packlink on 26th October 2021
I did not pay for "enhanced" insurance.
I dropped off the parcel (Boxed iPhone within brown "postage" box) at an InPost locker on 26th October 2021.
The Buyer told me (and thankfully sent me a lot of pictures) that the iPhone itself was missing from its original box, but that there was no obvious signs of tampering.
I contacted Packlink claiming for £700, after some back-and-forths they confirmed with me that their investigations discovered that my parcel was tampered with. They automatically refunded me £25 even though I made it clear I was after the full amount.
My next steps:
I will now Email InPost directly to make a formal claim. I will give them 10 days to reply.
InPost will more than likely deny responsibility and won't refund me. In which case, I will have to draft a letter of claim and begin legal process against InPost.
The Contracts (Rights of Third Parties) Act 1999 legislation allows me to begin legal proceedings against InPost (a UK registered company) and not Packlink (a Spain registered company).
I will continue to read this sub-forum.
Thanks again everyone, you have no idea how much I appreciate this.