Hi Chris,
I think I have found the same problem as you. I also have 2 of these sanders, both of which have a failed 'fan' unit in different ways. In one the armature has sheared off the fan and the other has your 'constant turning' problem so nothing can be attached properly.
I've resolved that both need the fan unit replacing. The problem being finding the things as they don't seem to be available anymore. I'm wondering if any similar sanders have a fan unit which fits.
I've found the following available:
https://www.sparepartsworld.co.uk/product/SPARE_BEW200%2FTYPE_1
https://www.mtmc.co.uk/--LE--/Black--Decker-Indoor-Spare-Parts/Sanders/Black--Decker-KA272-Type-1-Orbital-Sander-Spare-Parts__p-12736878-12736887-12736897-135535.aspx
https://www.partshopdirect.co.uk/black-and-decker-fan-sa-ka270k-ka272-sander-p477744-00/
I'm giving the mtmc.co.uk one a go - wish me luck!
