Hi Peter,
This is my first season of no-dig and Charles’ gardening principles, and I’m following Charles’ 2019 Calendar, so today (Valentine’s day) is the big sowing day for the first set of crops. I’m going to germinate the seeds indoors and prick them out into module trays, and the move them to my electrical hot bed in my unheated greenhouse. My hotbed is 60×120 cm and powered by a 300w heat cable burred in sand. Pretty much the same setup as you have. I’ve added a temperature regulator to my hot bed, as 300w is a bit hefty most of the times, and when the ambient temperature is mild (5-10 Celsius) the heat is only switched on some of the time. Charles’ hotbed is made of fresh horse manure, and I remember correctly the soil temperature in his module trays were about 20 Celsius (at least the day he made his video).
I see you heat cable is 25w, and I suspect this is a bit at the low end. Have you measured your soil temperature?
Regards
Erik