New modern wc just installed. Close coupled pan and cistern. Pan is
"back to wall" and cistern rests directly on top it. All connections
satisfactory except those joining the pan to the cistern.

These are not the standard connecting bolts and plate.

Two bolts go through the bottom of the cistern and screw into fixings
within holes in the back of the pan. They are designed to do this
without anything between the pan and cistern.

Supplied are conical black rubber washers (and metal washers on top) to
seal the holes in the bottom of the cistern. These leaked massively. So
I added thin plastic washers between the pan and cistern.

I have undertightened, appropriately tightened and overtightened the
bolts. One is fine but the other still leaks whatever I do.

What is the best way of sealing this connection? Fibre washers,
sealant, different conical washers (I have tried white stiffer ones
already) or what?

TIA