Remove the duff plaster, dry line then tank it. I'd suggest using Mapei P9 adhesive as its got a very long pot life. Mapei UCP grout is fine but it might be a bit quick for you.
Grey grout with white metros looks ten times better than white grout, in my opinion. You only need a very light grey, nothing too dark. Mapei Ultracolour plus silver grey or Tilemaster grout 3000 light grey.
Thanks Magga - If you guys are suggesting it I may just go for it. It would give the tiles more definition as well. Thanks for the grout suggestions too
Remove the duff plaster, dry line then tank it. I'd suggest using Mapei P9 adhesive as its got a very long pot life. Mapei UCP grout is fine but it might be a bit quick for you.