After almost 3 days of sitting in the room the cup of milk seems to have finally soured. Now I've added a cup of flour and wait again for 2-4 days to see if "yeast is in residence" and it starts bubbling. Bubbling, and not mould growing!
My crock of Sauerkraut is happily bubbling away in the hot water cylinder cabinet, no bad smells so far!
The trial with freezing Quark was successful, which means I can keep surplus there in readiness for baking!
Off to bake some biscuits now, the southerly front has arrived and we need comfort food.