I have attached an image of the spark plug - doesn't look too bad to me, but I'm no expert on reading them.
I had just pulled out the plug to check its appearance, and while I was doing that, shined a light through the plug hole to have a look and saw scoring, which prompted me to take the head off. There weren't any other obvious symptoms.
I did run it at a varying idle speed for 15 min or so on first startup, and then did a couple of hours of light work before the heavy mowing. But I probably should've known better and changed the oil before doing heavy work with it.
RLP
Last edited by RLP; 30/06/1906:59 PM. Reason: Add image, oops