Kimchi, Kefir, Sauerkraut and all other fermented foods

I should probably quote myself directly into the Guardian thread; but I bought one of those for getting a good temperature for Sourdough starter. Annoyingly whilst it says it goes down to 25°c it's actually closer to 35°c. Not sure if it's a fault or just the way it is. Otherwise a tidy bit of kit.

Edit:

but I notice that the 'potency' (activity I guess) of the culture definitely declines after two or three batches meaning you need to refresh the starter culture with more supermarket yog periodically.

Based on the above; I would assume that since yoghurt is fermented at a relatively high temp, you end up killing more of the bacteria than can reproduce. You could try taking a small amount out each time into a separate jar and feeding it with milk and keeping at room temp and using that as your starter