A few months ago I laid some slate tiles outside on a mortar bed, and
unfortunately one has lifted. Underneath the mortar bed is completely
flat and undamaged, and I'd like to be able to just stick the tile back
down with something, rather than hack out the old mortar (no doubt
having to disturb the surrounding tiles too) and replace it - with the
risk it'll probably lift again anyway?

Any thoughts as to suitable adhesive? Or is it Portland Time?

Thanks
David