How do I to fix a leaking dishwasher?

Simon, you might have a split hose or a loose connection. If the water in the house is turned off there might still be water draining from the dishwasher lines or sink lines--once the water is gone from the lines the leak will stop. You are going to have to pull the dishwasher out from under the counter to see what's wrong. If you aren't confident that you can fix the problem you should call in a professional.

Gk gives good advice. I would only add that the leak could also be coming from the supply line.

How old is the dishwasher? How old are the connection hoses? When might they have been checked to ensure that they are still adequately tightened?

Take off basket/filter in bottom of dishwasher and clean this area out well,dump baking soda and then white vinegar in same area let sit then followed by boiling water then hot rinse cycle repeat if still slow disconnect hose from drain/garbage disposal end and clean it out you can start by putting some baking soda and vinegar with a little Dawn dish to cut grease build up in hose end and just sloshing it around in hose(put your thumb over opening) then rinse cycle hot. repeat. don't put anything into hose like long brush because you don't want to crack it or poke hole into it. do this every few months with baking soda/vinegar by filter basket maybe monthly if you do not clean off food debris well before you load dishwasher.

Thanks for the support