I'm now tiling the ceiling of my shower cubicle, into which there will
be a shower fan/light inset, so a 4" diameter hole is needed through the
tiles. This will affect 2 tiles only, and I've already cut out two
half-moons from these using a hand tile-saw but have yet to attach these
two to the ceiling. These are standard ceramic tiles, ~6mm thick.

Unfortunately however, the hole's a bit tight for the fan, so I need to
enlarge the diameter by 1 or 2 mm - the sort of task I'd normally do
using a file or rasp were I working in wood or metal. But what would be
a good way of abrading a ceramic tile without busting it? (It took a
long time to saw the hole, so I'd rather not have to do it again!)

Thanks

David