I have just fitted a new toilet, where the cistern sits on the pan, there are are 2 bolts that have rubber washers, 1 is water tight, the other is leaking. I have tried putting mastic underneath the rubber washer, but it is still leaking.

How can I prevent it from leaking, any special washers, or special sealants, please help

Empty and remove the cistern. Remove bolts and dry the bottom of cistern well. Inspect around the holes for cracks or damage. Coat the rubber washer, top and bottom, with silicone. Reinsert the bolt through the cistern bottom. Install a brass washer and nut, and tighten securely. This will provide the water tight seal you need.

Reinstall the cistern onto the pan and secure using an additional brass washer and nut. Insure that the nut and washer used to seal the bolt in the cistern does not interfere with the seating of the cistern on the pan.

The risk of using the bolt with a single nut to both create a water tight seal and to hold the cistern to the pan, is that if you over tighten to create the water tight seal, you can crack the cistern. This can occur because there is space between the cistern and the pan. By using a additional washer and nut on the cistern to create the water tight, you can tighten with out risk of breaking anything.