Thanks Rob for your reply. Yes the plaster was sealed with a sealant for new plaster before you tile so assume that covers that angle.

So in summary of I understand correctly, the water may get the grout and the adhesive wet but ingress further would be stopped my either the plaster sealer or the Hardie Backer board. Also neither grout or adhesive will degrade because of the wet.

Is that correct?

Thanks again

Lee.