I have Welsh slate coping stones on top of my brick built garden wall. The wall is a double-skin red brick type and the slates on the top are about 30cm x 30cm. The slates have come off and need fixing back on but I have been given conflicting advice about how best to do so - one mate said that normal mortar is fine (and this seems to be what the original brickie used) whilst another suggested some sort of silicone would be better. As they are slate, the underside of the coping stones are pretty smooth, so there isn't a huge amount of purchase/grip for whatever is used, but the original mortar seems to have worked for the 5 years or so since the wall was originally built. Any thoughts/advice would be gratefully received.