Julie wrote:

Rich, thanks for response. The areas in question are spread throughout
the bathroom walls. Though none are within direct range of the shower
or taps, all are prone to condensation (it's a small bathroom, without
a window).

The builder has spoken in terms of 'making the grout into a paste' for
this purpose - could be that he's thinking of the nonsanded option.
I'll ask him - thanks for your suggestions.

I'd use an acrylic caulk in such an application--it'll go into very
small cracks much easier than a grout mixture (unless it's mixed so thin
it'll be very prone to shrinking as it dries) and will be quite
water-tight.