Hi Wee rabbit,
If you can't get hold of Bal superflex grouts, an excellent alternative is the weber/howtex fine wall and floor grout, you can get it in a matching colour, it's very soft on polished marble and easily scratchable surfaces, there is very little grittiness when applying it, and it is very easy to get smooth if mixed properly. I use this most of the time for my stone fixing, and never have problems with it.
If you can't get hold of Bal superflex grouts, an excellent alternative is the weber/howtex fine wall and floor grout, you can get it in a matching colour, it's very soft on polished marble and easily scratchable surfaces, there is very little grittiness when applying it, and it is very easy to get smooth if mixed properly. I use this most of the time for my stone fixing, and never have problems with it.