Many thanks for your enquiry concerning the soil warming cables. The proposed set up you describe sounds ideal and of course the layer of polystyrene beneath will help retain heat and germinate seeds efficiently. Although seeds and young seedlings need the bottom heat, covering the propagation bench also has benefits as the seedlings will benefit from the warm, humid conditions and you won’t have to heat your entire greenhouse when growing on.I’d certainly advocate the use of a thermostat to accurately control the temperature with any propagation system.
The clearer the material you use to form the cloche the better, as the seedlings will require as much light as possible to start producing their own food for growth; our clear polythene sheet fits the bill.
Best of luck with your propagating and vegetable growing this year!