In message , Chris
Howard writes
My extremely cheapo homebase own brand cordless drill has a keyless
chuck, which caught me out when I first bought it.

The chuck has a sleeve, which moves forward and backward, and locks -
i.e. you can't undo the chuck with the sleeve in the locked position.

Could it be something as simple as this?

You mean an SDS chuck then ...

(although I doubt that Bosch use the same chuck as my made in Taiwan
special..)

--
geoff