Tool setting datum shift

Hi Everybody,
I have a Bridgeport 1000/22 with a HH 426 control.
At present I set the tool lengths from the same work piece Z datum, but sometimes that datum isn't always available once I start machining and I break/blunt a tool.
In the recesses of my mind, I seem to remember that you can switch to another tool table/datum table (?) to set tools with and then revert back to the work piece datum system. I also seem to remember an M code to help on this but I may be confused with the 530 control.
Any advice on how to set tool length away from the job/work piece would be welcome. I have tried many different things but have failed. It must be possible.

Thanks,
RHK