Hi everyone,
I worked out how to do this. I created a block where I used the SimPowerSystems parts Variable Resistor, Solver Configuration, Single Phase Switch, Current-Voltage Simscape Interface, Electrical Reference and two Simulink->PS Converter Blocks to achieve this functionality.
The Resistance comes via a constant into a port and is converted from Simulink signal to a physical signal via the Simulink->PS block and adjusts the value of the variable resistor on the +ssc and -ssc side of the current to voltage simscape interface block. On the simscape side there is a single phase switch which has its control signal passed the same way as the resistance.
This can be then used to change the value of the fault resistance on the target using the setparam command without recompiling the real time target model.