If the other heater is working through the night ,and both heaters a supplied from the same board, then the supply to the board is ok.
Has anyone checked the fuse in the board. (fuse box)
If the Dimplex lads had it working , it has to be a fault in the supply circuit.
This is a pic of the board, my fuse board is the one on the left, it has 3 fuses, one for the ceiling lights, one for the sockets, the other must be the night supply the heaters.