Problem identified but not solved yet
The problem is the fan, what is happening is that it is slipping on the motor spindle and jamming against the metal cover (which does exists and is a sort of clamshell case secured by 3 screws).
Looking closer at the spindle of the motor it looks like the fan grub screw has over the years (15 odd) ended up notching a groove and so when the fan is secured in the correct place it is moving out a bit eventually and then stopping.
So fan is issue but I suspect that I cannot just replace the spindle on the motor and so unless I can think of another safe way to keep the fan in place (perhaps drilling a deeper indentation at the correct part of the groove ) then it will be a fan motor replacement just to get a undamaged spindle,
The problem is the fan, what is happening is that it is slipping on the motor spindle and jamming against the metal cover (which does exists and is a sort of clamshell case secured by 3 screws).
Looking closer at the spindle of the motor it looks like the fan grub screw has over the years (15 odd) ended up notching a groove and so when the fan is secured in the correct place it is moving out a bit eventually and then stopping.
So fan is issue but I suspect that I cannot just replace the spindle on the motor and so unless I can think of another safe way to keep the fan in place (perhaps drilling a deeper indentation at the correct part of the groove ) then it will be a fan motor replacement just to get a undamaged spindle,