Non trackable sorry.

Session prefix is from the server that printed the postage stamp details.

There will be no way you can prove one way or the other if the item was ever received.

Under the postcode if it says

"It is important that you retain this receipt as it is your proof of posting"

than that is all you need to start a claim on the 15th day after it was due.

A P58 form is what you need to start a claim

ftp://ftp.royalmail.com/Downloads/pu...rm_English.pdf

They will require evidence of the value of the items sent (value is not the invoice or sale amount) but your compensation will be upto a maximum of £39 (now) or £36 before the price increase.

They will also send out forms to the receiver who must state and sign he has not received the parcel before any claim is paid.