What's your plan with the tiles, are you refilling the entire area? You don't want to cut the tiles in situ, thatll make a horrendous mess. I'm no tiler but I'd suggest you fit your bottom 2 rows and then offer up the top tile back to front and strike off a mark from the shelf (remember to leave a grout gap). Then take it outside and cut it on a bit of scrap wood or whatever. This way you can flip the tile and have the clean uncut edge along the top, with the potentially ropey cut hidden behind the cistern. Do it individually for each tile because theyll all be different looking at that shelf.
Also if you're going to fix the cistern back through the tiles, get a 6mm tile drill bit, you'll struggle to get through them with normal masonry bits