That may have been me. Had a hell of a time trying to sort my lost ticket last year after it was delivered to my neighbour (who kindly delivered it to me, opened, week after glasto).
Like others had said OP, get written confirmation from Royal Mail saying it’s lost. SeeTickets won’t accept anything less. And if you could do it all last week, that would help. SeeTickets took best part of 2 weeks to reply to me.
Good luck OP!