It's a low pressure connection - just the weight of the water.
Perhaps a new donut washer but before that check the wing nuts either side - pointing down at the very back of the bowl - tighten them to put a bit more pressure on the seal.
Also - squint sideways at the narrow gap between the cistern and the bowl - it should be parallel - if not make it so.
Sometimes when the cistern gets put back it doesn't sit square - front to back. If this is the case a leak is bound to happen.
Good luck...
Peccavi, April 2013
