Returned Phone To Samsung, Samsung Claim The Phone Didn't Arrive

[LiamRainbow]

I bought a mobile from Samsung which came with two free items. I hadn't used any of the products and wanted to return them.
I cancelled my contract with Samsung and agreed for the phone to be collected on a scheduled date. The courier failed to show up on three separate occasions. Two weeks had passed by the time they collected it!
I returned the phone to Samsung, using their postal service via DPD, in-line with their returns policy. I sent the proof of postage to Samsung. The box was then signed for by Samsung a couple days later.
The free items were 'refunded' by Samsung, but no refund for the phone. After a month of back and forth emails, Samsung claim the phone wasn't in the box.
I contacted DPD, who said 'Samsung need to contact us to launch an investigation, we will be able to tell at which point the phone went missing due to the weight of the box'.
Samsung are refusing to contact DPD and are refusing to refund me without me proving the phone was in the box (which is of course, impossible).

• 14 days had passed since I cancelled my contract with Samsung, and they hadn't refunded me. Shouldn't this alone mean they're in breach of contract?
• Samsung and I agreed in writing that the phone will be retuned, and they signed to say they received the package, will this cover me?
• I paid via PayPal and will launch an investigation, but they've been horrendous to deal with in the past. Any tips?
• As Samsung arranged and paid for the return of goods, am I even liable?

Thank you to everyone who reads and helps me with this matter. I've been stressing over this a little bit too much, I can't afford to lose money and not have the product!