I doubt that it’s possible to cut a nut so precisely for 12s that would bind with 13s, especially as nut slots would tend to get slightly wider with use.

My D18 spent 40 odd years with 13s but I gave it a rest with 12s several years ago.

Picked up one of my guitars from the Guitar Repair Workshop (Hyde branch) yesterday. They’ve had it for three weeks - friendly bunch and a relatively painless process - much less hassle than travelling to their previous premises at PMT in Salford. Sounds like they intend to keep both this and the Manchester branch operating.