How can I fix a water leak between the cistern and toilet pan?

Between the two elements - pan and cistern - is a soft washer called a doughnut (donut) because it looks like a doughnut. A metal plate presses down on it - it forms the seal. The metal plate has two wing nuts to apply the pressure. You can reach the nuts - one each side below and at the rear of the pan.

Youtube has videos (donut washer)

If that doesn't work then call a plumber.

If you have the confidence you can do the job yourself - remove and empty cistern - fit new washer - put it back together - about 30 mins.

You can see the arrangement if you squint along the join from the side - the cistern needs to be flat and parallel against the pan else an effective seal is difficult.

Good Luck...
March 2016