Robert. I used 41 x 35 kg bags of pre mixed concrete that is 30mm± thick, about 14.2 m². Actual cost was €190 because we under estimated the concrete coverage but still a LOT cheaper than buying ready mixed concrete
Vern, the base is shuttered with timber that has a slight give in it on 2 sides. There is a one piece plastic membrane inside and re-bar over the majority of the central area. The white cable is indeed an electrical cable in a trunking, laid under the membrane in a floor channel. It is for power to my saw and router tables and chop saw.
Rather than take everything out of the workshop I decided to just install the new floor in the centre, leaving the benches round the edge on the old floor. Anything stored under them sits on pallets anyway so there is no problem with damp in the winter.