TheOldFellow brought next idea :
My first experience with one of these has not turned out well. Granted
it was a cheap one from Toolstation, but the core itself is not
concentric with the Guide drill, nor with the arbour. So I can't get
it stated.

Is this usual for these things?

Yes, probably a manufacturing defect - if you are certain it is
correctly assembled?

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk