Thanks to all the good advice I received here I finally managed an entirely staisfactory sourdough loaf. The full story is at my blog, but the bare essentials are that I made a 60% hydration dough with 100% strong Manitoba flour. The starter was 10% at 100% hydration. I stretched and folded during about three and a half hours bulk fermentation at a hot room temperature, then formed the boule and let it rise for an hour or so before putting it in the frdige overnight. I baked from cold, into the hottest oven I could manage. And the ugly looking scoring was achieved with a pair of scissors, snipping the dough four times.

Thanks again.