Many thanks for the rapid response, and for confirming with your own checks that the problem does not lie with Ivory Search.

I checked through my plugins, and after a few experiments I found that the Relevanssi plugin was not recognising strings formatted like my reference number. When I deactivated Relevanssi, Ivory search returned the full reference number without a problem. For now I’ve deactivated Relevanssi, and I’ll speak to them about it.

Thanks again.