Tilers - I've got a hole in my slate tile!

I would use epoxy resin, should work as it's good for brick etc. A bit messy to work with but that's what I would try.

 

If you already have some, then sand and cement should do nicely. Fill the void right up, and then level it so's it takes the tile piece at the correct height.

It should adhere fine, although it might be tricky to get it to the exact right 'level'. Ie - the tile chip will likely be a perfect fit to the area it came off, and you'd want it replaced the same way. Sand/cement is too gritty and thick as an adhesive for this tile-to-tile mating, so by all means use a smear of epoxy - or even 'super' - glue to fix the actual chip down in place afterwards.

But I'd still fill that void with something solid like sand/cement in order to support it and prevent any further damage.

 

The void is around 50mm x 50mm - but how deep? If it's shallow, almost anything will fill it - you could use patching plaster, for example, provided you let it dry out fully.

I mean, it ain't worth buying a bag of mortar for the need for a tablespoon's worth.

If you have tile adhesive, then even that will do it. If it's the type that dries out to 'set', then give it a good week to do so before fitting the missing piece over it. Or even fill the hole and 'tamp' the piece down on top until it's flush, and then wipe away excess adhesive using a damp cloth. Place something over it all for a week so's it ain't disturbed.

 

This was my shout also.
Wear a mask when dealing with epoxy, this is in case you don't want some "accidental fun", if you do, well don't wear a mask.