Hotpoint WD860 Heater fan problems

I have a Hotpoint WD860P (s/n 49147485 – I have had the machine for just under three years) that has a recurring problem and I would like to know if there is something I can replace to solve it once and for all.
I know this is a common fault because I have purchased a new dryer cut out and thermostat before so you can guess where this is going.

If the dryer is on high heat, within 20mins the dryer fan slips down the motor spindle and hits the dryer box, fan stops, heater over heats, one shot trips blows, damp washing.

The problem doesn’t exist if the unit is on low heat. It is becoming very boring having to open up the unit and shove the fan back up the motor spindle when we forget to change the heat setting. I can’t see a way of tightening the locking nut or any other way of preventing the fan slipping. Trying to turn the nut to tighten the fan on the spindle just leads to the fan turning around the spindle. (am I doing it wrong?)

I did read on a forum somewhere that Hotpoint revised the heater box arrangement but I don’t know whether my machine is affected by this or not.

Any advice on what to do or a list of bits that can be replaced so that I don’t have to keep taking the top off would be appreciated otherwise, next time it goes pop will the point where it gets recycled or added to land fill.

Thanks in advance.