Let's just say that if I ever have to remove a tree stump again, I shall be doing it properly with a grinder.
I elected to do it this way because I wanted to get rid of it a lot deeper than 20cm, (this is nearly a meter below the original ground level), so I had to dig anyway, and after I dug I thought I'll just cut it with my chainsaw then.
But yeah, dirty wood and chainsaws don't mix. Lesson learnt.
However, I am happy to report my latest method is working nicely. Thanks @tallsam for the inspiration.