John. See various out macros on the following link.
http://topas.dur.ac.uk/topaswiki/doku.php?id=macros
Something like the following should do the job.
out "results.txt"
Out_String("Rwp")
Out(Get (r_wp), " %11.5f"\n)
Out_String("Parameter Name") ' the name you want to call a parameter
Out(parameter_name, " %11.5f", " %11.5f"\n) ' outputs value and error of parameter