It Has Been Decreed that we need a new house number on the drive. She
bought three ceramic tiles each about 3"x2", each displaying one of the
three digits of our number.

Now, the wall to which these are intended to be attached is a drystone
retaining wall on the pavement, so not exactly ideal. How best to do
this? I think it would look least ridiculous if the tiles were attached
directly to the front of a single stone, rather than mounting on a plate
or something. There's a reasonable candidate stone, which is fairly
flat-faced but rough, and currently covered in lichen etc.

I'm thinking pressure-wash followed by some form of adhesive but what?
Waterproof tile adhesive? Car body filler? They'll need to be damned
well attached because as well as weather constraints, we're on the route
for kids ambling home from school, for whom the obvious addition of a
bright shiny number at pavement level will present an instant challenge.

Or shall I just advise SWMBO to buy a pot of white paint? (Hint - no)

Thanks
David