how can I fix Dimplex XLT6N cutout?

Two months ago I replaced a 25 year old Dimplex XLT6N (0.85kW) in the bathroom with the model recommended by the local supplier, the Dimplex XL6N (850W).

Two problems:
- the new one gives only half the heat as the old one.I note that the instructions describe the XL6N as 6 kWh not 0.85kW

- yesterday the heater failed to come on. It is possible that the cutout switch has operated, but I cannot see how to reset it. I have taken the covers off etc. but there is nothing obviois to flip back as in the diagram in the instructions.

Any thoughts before I take it back to the supplier for a refund.