Lap #3, (lap #2 still too hard) followed GW's instructions to the letter in preparing the lap. Lots of cerox, cold pressed for 2 hours, now I've locked it up and no amount of pushing/pulling (1 metre long lever) will move it. In my haste I was going to seal the back of the plaster tool for another day, so now if I toss it in the bath for a day I will ruin the plaster.
Do not want to make another tool!
Remedy will be to seal the tool in situ with resin or enamel paint, then dunk it.