OK I've given up on that base stl and just grabbed a 40mm thread and started again. IT was too much work to correct the original model as when I loaded it into tinkercad to see what another CAD system though most of the hand grip disappeared. Going forward I'll just use the original plug where I need a blanking plug in my small suction system for a mini wood lathe.
Cura is showing the holes in the new model in preview and I'll get a couple printing later today / tomorrow when my printer has finished to job that it has on the go.
Thanks for all your help.
I will be leaving a comment on the plug model on thingiverse to inform the model originator of the problems that it has - if he doesn't know of them already.