I had faulty thinking about what the CONFIGURATION setup screen in the Melabs Serial USB programmer does. It appears to show the configuration that will be in force AFTER the programming step. Setting Data EEPROM to 'protected' does not protect it during the programming but it seems it will be protected when the program runs.

What I found solves the problem is to dig deep in the options\more options\ erase and options\more options\blank check to get the programmer to accept the fact that the chip was not blank and to program anyway and to not touch the data EEPROM during programming.

Pretty obvious after the event. Thanks to Jerson for pointing me in the right direction.

Thanks
BrianT