I am running Mach3 under Windows XP and its been running successfully for many years.
A problem has now occured where on starting a program run with a tool change setting at 50mm and work surface at zero the machine runs to z50 and immediately on stopping the Mach3 dispaly goes from 50 to 25.4, thus losing the datum setting.
I have done a number of restore settings and run previous successful programs as air cuts but they all do the same.
So I have concluded the fault is within the Mach3 software.
I have registered a Ticket with the help desk but as of writing this  I have not had a reply.
Is it worth reinstalling Mach3?

That's probably all you can do as Mach3 is now out of support for 10+ years. Save a copy of your xml file before uninstalling.