How can I stop my pavlova from "leaking"?
The most common reasons I know are:


Preparing when humidity is high.  Tough to avoid when you are on a schedule like wanting it for an occasion.  But in my experience, having an oven that circulates fresh air in can make this worse by bringing in more moisture all the time, or ever opening the door or trying to cook something else at the same time.
Over beating can cause it.
Undissolved sugar, which of course is opposed to trying to not over-beat.
Overcooking.


A couple things I do see in your recipe:  I am used to seeing a bit of vinegar.  I assume the lemon is to cover the same with a bit fresher flavor, and it may but it might be worth trying such a recipe.  I know the point it to help create the classic Pav crust while I am used to lemon in pavs being more to help the foam form buy your recipe calls for it after you already have peaks.  

The 2nd thing I see is your recipe calls for a bit higher temp than I am used to.  For instance the first I checked to refresh my memory called for 190F for 5 minutes, then drop to 130F for an hour, then turn off.  I think what may friend from NZ who made much better than I do used was 200F for about 10 minutes, then just the light that kept the oven at about 140F for an hour.  That is quite a bit lower than yours, so I would question possible over cooking as the issue.  

I have even seen the higher pre-heat skipped and instead torch the meringue and only do the low heat to slow cook the center, but when I tried that I got the nice cruch, but raw egg inside and it also leaked, but it was a egg white leak.  Yours visibly looks more of a sugary leak to my eyes.