The Acrylic tubing arrived so I can continue with the plinth. The tubing is I.D. 8mm x O.D. 12mm so an 8mm bolt will just slide through. The tube is used as a spacer for the stainless nuts/washers and an 80mm long csk machine screw is inserted from the underside to pull it all up nice and tight. The spacers have been adjusted so the top of the nut is just below the top of the mould and these will have some blu tac or similar stuffed in the top to stop cement getting down the ends. When the concrete sets and the screws are removed, the fixings will be anchored in the concrete and an M8 speaker spike can be inserted. By cutting a slot in the end of the spike with a hacksaw, the spike can be adjusted by inserting a long thin screw driver down the tube, who's open end will be on the top face of the plinth. By running a bead of silicon sealant down one side of the thread on the spike and rubbing it off with a finger, leaving just what is in the thread, you can make a form of thread lock. Leave the silicon to fully dry then insert the spike into the nut, you will need more than finger pressure to turn the thread, but once adjusted to the right height it will stay put without a lock nut and also without any unwanted vibration caused by chatter in the threads.