Adhesive for slate tiles on outside doorstep?

Think most decent exterior stone to stone adhesives will be a variation on mortar. So most probably a powder you mix with water. Most decent DIY stores will sell it. Look for a tile adhesive suitable for exterior use.

Gripfill. Works for slates on the roof that come loose, so no reason why it shouldn't work with slate on a step provided the slate is well bedded and doesn't rock. Just wire-brush off any loose stuff from both step and slate first.

Gripfill.

On 18 Jul 2015, "Phil L" grunted:

OK, interesting. Even those sold specifically as floor tiles, then? I think I'll just reuse the ones I've TBH, as they aree already cut to size etc and as I mentioned, am just tarting the place up for sale anyway.

Thanks to everyone else for the Gripfill advice - am off to Toolstation! :)

Is this a decent alternative to replacing or re-securing a slate 'properly' using straps/nails?

Just feels wrong . . .

My feeling too, but, about a year ago, I used Stixall to glue down a 4' high x2' square angle iron bench to slabs, outside. It's still secure and has been used for many jobs. Somehow I still don't really trust it.