I'm starting to wonder if the darkroom bag is softening the pressure too much and stopping the chemicals from spreading out before they harden. After having left the prints to 'dry' for a few days, I can see light cream/yellow patches all over them.
I think my next plan is going to be to make up an aluminium bracket and two steel rollers, with one "driven" roller and one "static" roller. The static roller will be pushed against the driven roller with an expansion spring on each side (probably a nice, strong one). Add a cheap gearmotor to drive the "driven" roller, job done.
Or I have a laser printer fuser assembly somewhere. The rollers in that would probably do the job.