Martin,
thanks for your comments and link to the calculators I shall have a play with these. I have read that road traffic vibration frequencies are about 7 Hz. I have seen some discs that have threaded bolts isolated from each other within the discs so that I may be able to bolt down onto the pier and also bolt down the mount adapter plate so that the system is isolated with the mount resting on 4 such pads. There are rubber discs of various sizes, the Sorbothane of this type are smaller.
Eric