Dave

when you say moulded base is this the substrate which has been tiled on or is it just the ply. if it just the ply then it should have been tanked to make this water tight, flexible adhesive used and a flexible grout , these products are water resistant not water proof so its sounds as if you have water ingression in the substrate and will need to lifted and done properly. i.e. any substrate to be tanked ( water proofed )