Diagonal floor tiling - how to start?

Well I'd start with a base line with the line running so that it looks correct from the door and possibly taking into account the relationship where the tiles come against other walls or straight lines opposite the door. Then work from the center and one side of that line to the edges.

The cheapo diamond tile saws have a an attachment to hold tiles to cut diagonals but I doubt it has much practical use in the real world as walls are rarely straight enough. Mark and cut each tile to fit where it's going.

Having done our en-suite on the diagonal, my answer to your question is "Don't".

If this is a kitchen floor you may not need to cut many, as the edges will be hidden under kitchen units. However, do make sure the surface is sound and hard, as most of our diagonally laid on wood plank floor tiles, are now broken and come adrift as the floor underneath was just too flexible for them.

From doing the kitchen the same way. "Seconded".

Even worse erindoors is now doing a terrace the size of a small African nation with herringbone tiles.

Amusingly expensive.

The job's done and looks pretty good. It was nearly as easy as tiling parallel to the walls but took longer because of all the cuts. Also, because virtually every cut was at 45 degrees to the edges they all had to be done on the wet cutter, rather than the splitter. Overall I think it was worth the extra effort. I snapped a chalk line to start from and then worked out from tiles on the line, the first positioned to avoid any ridiculously small triangular cuts at the edges. Using brass-coloured metal L section worked well to create a rug pit. The only tricky bit is that the tiles (B&Q) have a rough surface and this seems to hold the grout - it's taking a lot of wet/dry polishing to get them to look as they should. Thanks for the helpful suggestions.