your leak is coming from the releif valve, when the pipe is blocked the relief valve bleeds the pressure to prevent pump damage. the valve is located in the engine bay next to the fuse box cover, removing the cover and the water proof rubber lining behind it makes access easier. the valve is a black plastic parts which joins two pieces of pipe, one black and one white, the white pipe goes to the pump, the black goes to the rear washer. when you spray water and there is a blockage the water is released from this valve and finds its way down to the floor under the car making it difficult to locate. this is a common problem i've fixed a few times already. i would blow compressed air through the pipe (when its disconnected at both ends) and then remove the valve and replace it with a simple join pipe like you can buy at a pet shop for aquarium airpipe joining.