I bought a brand-new Breitling watch from authorized retailer for £4,165. The watch was faulty, so I returned it using Retail ’ authorised Royal Mail Tracked 48 return service.  
I posted the parcel at my local Post Office. I have proof of postage showing a weight of 500g.  
Royal Mail tracking confirms delivery to the retailer.

The retailer then told me the watch was not inside the parcel and a fake apology letter from Royal Mail was inside the parcel and refused my refund.

They also say Royal Mail found no recorded damage/tampering and that an alleged “Royal Mail apology letter” in the parcel was not genuine. I have no knowledge of this letter and did not put it in the parcel.

I have photographs from them showing what appears to be an issue with the tape/sealing around the barcode area.  
I have now started a Money Claim against the retailer for the £4,165.

The retailer have told me that the matter was reported to the police, and there has been a police investigation/reference relating to their allegation.

My main question is: what evidence should I be asking Royal Mail and Beaverbrooks for? In particular, how important would the exact weight recorded by the retailer when they received the parcel be compared with the 500g weight when I posted it? The retailer declined to mention the weight that has received despite asking several times.

I’m now wondering whether I should also make my own police report, because from my perspective the watch disappeared after I handed the sealed parcel to Royal Mail. I’m not accusing any particular person because I don’t know what happened.

I’d really appreciate advice from anyone experienced with Royal Mail, high-value returns or UK small claims.

£4,165 Breitling disappeared after I returned it to Authorized retailer looking for advice