To be honest I'd put them on top, the whole idea of the cellotex is to keep the heat in,putting them underneath will only force the heat to travel down.
If you are using plastic pipe for the rads, then i'd recommend using pipe clips thast will keep it in place,they have a jagged edge which will hold into the cellotex.
A final bit of advice would be to ask your builders merchants for a bag of fibre which you can add to the screed which will give it stregnth-escpecially where the heat from the pipes will be.
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