Johnson & Starley JB40-50 MAF 2 Warm Air Unit not operating correctly?

This unit has the Modairflow 2 control which runs the fan from very slow to full speed using infinitely-variable speed control. When in the 'normal' state, where a constant but small amount of heat is required, the burner lights, runs for about 40 seconds, then goes out. The circulation fan starts running at a constant speed rather than ramping up gradually (same speed as the ventilation speed selectable by a switch on the electronics module), usually a few seconds before the burner goes out. This runs for about a minute and a half, and then stops. The cycle then repeats, with varying time gaps. Sometimes the fan restarts without the Burner having started. What used to happen was that the fan ran almost constantly, at very low speed, with the burner coming on and off regularly. This gave a very comfortable feeling and seemed to heat the house very well.
The Thermistastat seems to be giving about 6.3V to the controller when up to temperature, this falls when the temp cools or the thermostat is manually increased. The burner lights in response to these thermostat movements.
The Air flow detector reads about 3.5K ohms when disconnected. The fan will not start at all if it is not connected. Shorting it out completely runs the fan at top speed.
This all seems to point to the R005 electronic control module, but as these are about £170 I am a bit hesitant to just order one without better diagnostics. This fault does not seem to match those in the J&S Installation, Servicing and Commissioning instructions ZZ384/1.