As far as I'm aware (04 DE Zed), passenger side reservoir is for headlight washer and driver side reservoir is for windscreen and rear. If it's leaking somewhere, I'm assuming the pump is working fine?
I think the headlight washer reservoir is connected to the main reservoir, as I only need to top up the main reservoir. Looking at the diagram above (since all the windscreen pipes are on the driver side), looks like it's leaking from the headlight washer maybe? Also check both 'Check valve A' and 'Check Valve B', if I remember correctly, they are under the brake fluid cover. Remove the valve A and test washer to see if water reach this valve. If they are blocked or faulty, when you operate the washer pump, the pressure might force water to leak at the weakest point. You can buy a simple connector (8mm I think) without the non-return valve (Halfraud stock them).
If you failed to locate the leak, I think you'll need to remove your front bumper or if you on a ramp, remove the undertray as most of the pipes will be there.
note: Pic from US service manual, (section WW - page 33). UK model might be different.