However, I would be surprised if any ticket office had any idea how this should actually be done as there is no process that I know of for doing it other than for a season ticket or railcard (and in over 20 years I’ve never done across such a situation).
I've no idea how it should be done, and no real knowledge of ticket issuing processes, but in the absence of guidance, I'd suggest
- phoning up someone who might know (head office?) for further guidance but if that doesn't work
- following the instructions for reissuing a season ticket or railcard and see if they work, or (assuming that doesn't)
- issue a new first class all lines ticket with the same end date as the original but with (a) no amount charged (I'm assuming that like the standard till that you'd find in your corner shop there is some arrangement for zero price sales) and (b) a
very detailed note for the auditors explaining why something so expensive has apparently been given away rather than sold.