Well, I've worked through the whole batch now. I've also changed (one at a time) developer (three different ones) stop (plain water as well as acid) and fix (refiltered, fresh and even plain hypo) without any improvement. The only thing I can't change is the tap water that I use for washing.
Also in the batch was one very old slightly fogged Kodak Bromide paper, and a few using only slightly less old Ilfobrom grade 2. They were stored in the same bag as the Kentmere prior to shooting, shot in the same holders, and then stored in the same bag as the Kentmere before development. Yet none of these exhibited the same spotty problem!
It's as if the Kentmere had got contaminated in some way; as I say, I made plenty of regular enlargements from the same box without seeing the same issue, so it can't be a manufacturing problem.
I'll just have to chalk it up (or down?) to experience, I think. I've run out of it now, and I prefer other papers anyway for enlarging.
They do contact OK, though, but it's a shame the scans from the negatives look such a mess.