There are situations where the sending and receiving of mail is legally preformative. Examples include Service (well explained in this answer) and unpaid parking charges pursued under the Protection of Freedoms Act.
My understanding is that all that is required in these situations is for the sender to say that they sent the letter. If the receiver did not receive the letter there is no evidence generated, and the letter is considered received if it is reported as sent.
Suppose the receiver of the letter had created and installed a device that recorded all mail received, perhaps something to ensure letter pass one at a time and a camera taking an image of each letter. Would the output of such a device be admissible evidence as to the non-receipt of a preformative letter?