Advice needed - damaged exterior tiled sill

[csemann]

I have bought an older house with tiled outdoor sills. I'm trying to restore and the actual tiles are in fairly good condition when the paint is removed. However the mortar between the two layers of tiles is damaged on the front of some. Can I ask what I would need to use to replace this with? I'm new to outdoor DIY and can't afford to pay someone at this point or replace them so any advice on what best to use would be appreciated! Thanks!

 

[blup]

Bostik cementone, screwfix do a rapid setting 5kg tub, experiment to get the consistency right. You can add sand if you want to thicken it out a bit.

Blup