Thanks Niko
I'm hampered unfortunately by geography and err...I think I'll call it aesthetics for now (what can I say, other than my overriding instruction has been "If you get in the way of my nice view I will make you invent a cloaking device"
). My property has a gully running right through the middle of it, which is basically a moisture trap so I can't go further down the property or I'll end up being a participant in that Monty-Python sketch where they build the castle in the swamp, can't build off to the side or in front (driveway in front, large shed to one side, house to the other). The options are not good
You are right of course, the top of the tank is going to give some vibration. My only hope is to mount the pier in a way that dampens it all, which I have some flexible tile glue and a piece of marine rubber matt. I'll make a sandwich of that in between the tile adhesive and hope that does the trick (along with bolting it in to the slab which is only 200mm thick - you are correct there). The pier is concrete filled, which will reduce vibrations to some extent.
Peter I appreciate the offer of diagrams, I admit I am struggling with the best way to set this up - but cannot actually get in to the middle of the pier to fix a centre bolt.
I thought that something like this might work?