Thanks for all your replies. I can set them out correctly but I would have liked to have nearly 4 full tiles from the shower tray to the wall instead of 3 and 2 near halves or 4 and a narrow cut. I just thought I had cracked it that was all and if it was unusual to get a 6mm tolerance I would complain and see if I could get a batch nearer the stated size.
As it looks like my luck is out would you go for a 20mm slither against the opposite wall where ther is a WC, wall hung basin and Rad and have a full tile against the tray or a near half against the wall and the tray ( which hits you as you go in the room ) ????
Any suggestions would be appreciated