Thanks for your reply.
The fan isn't visible at the rear of the freezer once I've removed the drawers and I can't see where it may be, so I cannot see whether it is rotating or not. When I switch the freezer into 'continuous operation', the orange LED lights up and there is a whirring noise. When put back into 'normal operation' the green LED doesn't light up (the instruction manual suggests that it should), no whirring noise occurs and the freezer gradually warms up.
Yes this was my first thought. I had first turned it up to 3/4 of the power and experienced the same problem. I have now turned it onto the maximum setting this evening, so I guess it needs a little time still to rule this out, although I was surprised that at 3/4 it wasn't keeping the almost-empty freezer to temperature.