Ah that's the problem with having time away from here, had an unexpected visit this afternoon from my woman. I suspect she was checking up on me ;-)
But it seems you have some good suggestions anyway. It's not something I've had to do but my usual first port of call is to see what the local DIY sheds have. Or better still, a tile specialist shed if you have one nearby. If the tiles and upstand are already in place, then what sort of gap are you hoping to fill ? Might a silicon rubber filler (or even a grout) not patch any gap ?