( apologies for the pun )
The sole reason for the odd behaviour of my trusty SCR speed controller with the new jigsaw was due to the suppression cap fitted across the AC leads of the saw - 220nF in this case but 22nF or 47nF was just as effective at preventing operation.
Removing the cap produced normal speed control.
Investigation showed that with the cap in place, large bursts of ringing voltage appears across the motor each time the SCR switches off - this interferes with the operation of the diac firing circuit and hence the drill will not run.
There is a very simple fix that allows the cap to remain: using a double adaptor, connect a 100/150 watt lamp in parallel with the saw or other tool fitted with such a suppression cap.
Damps the ringing and normal operation resumes.
See ABSE for schem of the controller - an EA design sold by DSE as a kit long ago.
....... Phil