Do the tiles when fitting to the surface have to be cut at the tangent where the fall to the drain is?
That's correct. You must cut your tiles according to the premachined slope on top of the tray.
There are diagonal slopes from every corner of the tray to every corner of the drain and your tiles must be cut according to that. Please see technical diagram on one of the information tab, it's shown there.
I am happy to assist if you have any questions.
David 020 7193 0953