It sounds as if it might be a physical problem rather than a software/firmware problem.
By that I mean the circuit that starts the fan in the 'off,low,high' setting does not appear to be switching when it receives the start signal from the temperature sensor. That part of the circuit is not operational in the 'auto,low,high' setting.
The temperatures you are seeing are good for long life of the drives and, yes, £5 is much better than new drives and unless it is very dusty where your unit is installed you most probably won't need to pay that out for several years (we have a couple of DNS-323 boxes that are set that way and they have been running continuously since we bought them when they were first available, actual date lost in the mist of time).