I tried drilling the retaining screw out but it broke three drill bits but couldn't tell if I had the whole head off - I hadn't... In the end I used a 24" stillson which stripped the thread off 10mm of the end of the screw. Have re-tapped the motor stub using the cut down screw and have successfully fitted the new chuck. The screw is as expected the opposite to usual thread so I couldn't use my tap and die set!
For the record the new screw that came with the chuck is a phillips countersunk head.