Thanks to everyone so far.

Chris - the connection is exactly as you describe, except the bottom
nut is actually built into the hole in the pan.

Roger - the doughnut works fine. It is actually a special very soft one
supplied with the wc. Somewhat thinner and softer than the usual ones.
However I can verify that the leak is from the connecting bolts - not
the doughnut.

I agree the plastic washers shouldn't be necessary.

The hole in the pan doesn't appear misshapen. I suspect the fault lies
in the shape or quality of the conical rubber washer.

Set Square - I am sure. The nuts under the cistern do exist - but they
are incorporated into the holes in the pan. It is a modern designer
type of wc - there is a flat bottom to the cistern which sits exactly
on an exactly equivalent flat ledge on the pan. The pan extends right
back to the wall. There is no room for anything in between. Originally
I tried larger plastic washers between pan and cistern - but the
cistern became unstable - all of it's bottom sits level on the pan.