In case this is later read for information by someone in the same boat,
this issue is now solved. Basically the piece of wood in front of the
truss rod slot was either not quite accurately placed or the hole was
not big enough - it certainly is a minimal hole. The truss rod slot
which thankfully turns out after all is for a standard hex/Allen key
with a 3 or maybe 4mm diameter end had not quite been accessible. With a
small gentle bit of twiddling with a file and then a bit of fine
sandpaper to clear up everything is fine, although the rod is stiffer
than I'm comfortable with. It's a shame this slot in the wood wasn't
checked before the guitar left the factory but anyway, anxiety over.