Switch the appliance off at the machine and at the isolation switch (usually labelled and above the work top).
If the leak continues, look for the small blue and red taps, usually found under the sink, and give them one quarter turn to the off position. These are isolation valves, and are in the off position when pointing at a right angle to the pipe on which each one is fitted. Some valves have a handle whilst others have a slot for a screwdriver or small coin to turn on/off.
Try to mop up as much of the water as possible. You may need to pull the appliance out from the wall to access the taps fitted behind it.
If water drips from a ceiling, place any available container (bucket, saucepan, mixing bowl, pint glass) underneath to minimise the damage.
Then call Leydon Lettings on 01227 713913 (office hours) 078 345 404 57 (out of office hours).