I've not tiled a roof before but have a pretty good idea of what's involved and have been reading up. One or two guides say the use of tile clips is important on verge tiles even when mortaring them.

I'm using Marley Eternit Ashdown clay tiles but Marley don't seem to mention clips in their fixing guides for clay tiles. Been on the phone to them and they don't make clips for the Ashdown plain clay tiles.

It's a moderately exposed location but not exactly the top of Beachy Head. Are clips used for plain clay tiles or ?

Mechanically fix (screw down) all verge tiles and all eaves tiles (bottom two courses).  As has been said, only use full tiles and 'tile & ½' tiles, never halves or cuts on a verge.

I use copper clout nails if the batten is fully fixed and doesn't flex, otherwise stainless screws like the ones you mention. Don't tighten them up too much though!