Very odd - the original layer of tiles were stuck direct to the
brickwork with mortar, then evidently the plaster was applied, bringing
it up to the same level as the tiles (you could see where mortar had
splayed out from under the tiles, and was concealed by plaster). I
think this stretch of tiles was probably running the length of the
original bathtub; that's my theory.

our kitchen was like this - tiles bonded to the bricks with mortar about an
inch thick (about halfway up the wall), then plastered the rest of the way
up to the ceiling. A right pain getting it level after taking the tiles off!