My mum manages it without fail.

She uses a sort of "bain marie" but without additional heat i.e.:

• 1/4 fill a deep bowl with hot water from a kettle and let it cool for a few minutes
• put the yoghurt in a smaller bowl and rest the bowl in the warm water
• put a plate over the bigger bowl so that some of the warm water vapour circulates, and also warms the smaller bowl
• leave overnight

Good luck.

My mother uses a sort of "bain marie" but without additional heat:

• 1/4 fill a deep bowl with hot water from a kettle and let it cool for a few minutes
• put the yoghurt in a smaller bowl and rest the bowl in the warm water
• put a plate over the bigger bowl so that some of the warm water vapor circulates, and also warms the smaller bowl
• leave overnight