Lost/missing half a UK rail ticket (advanced purchase), can I get it reprinted?

A few weeks ago, I needed to book tickets for several different journeys, over a few weeks. It seemed to make life easier to book them all in one go on a single (no fee!) website, so I did. Consequently, when I picked up the tickets, it was a single booking reference which lead to about a dozen tickets and seat reservations being printed out.

I'm now about to do the final journey from this booking, which is an Advanced Purchase ticket. As such, it comes in two parts, one part being the Ticket, the other part being the Mandatory Reservation. It's only valid if you hold both parts, needing both the ticket which says what route, and the reservation saying what train + what seat on that train.

The snag is I discover I now only have one of the two parts. I don't know if one part didn't print properly (I did get about a dozen bits of card in one go, between tickets and seat reservations), or if I lost it (eg it fell out when I was getting a different ticket from my ticket holder), but the upshot is that I don't have both parts.

Is it possible to get the missing part (re)printed at all? Perhaps from a station ticket office, by showing them the part I have and the booking reference? Or by speaking with the website I bought it from?

(Normally I gather they won't reprint flexible tickets, presumably due to worries that you could otherwise give the duplicate to someone else to use, but as this ticket is a mandatory two-part ticket, someone with the missing other half couldn't use it)