Dave

hi there welcome to forums. firstly if damp has ingressed into the wall it could have damaged the plaster by which i mean when it got wet it would have softened and then as it drys the plaster becomes unstable i.e. crumbly, i would remove tiles until you get to were the wall is dry , remove any damp plaster and patch back in with one coat plaster this dry's rapidly when completely dry then you can re-tile, not much point in tanking area were tiles come off unless you are going to do whole lot, use a siutable adhesive and modified grout, re-seal around the bath and could also be worth while re-grouting all the tiles and give it a fresh look.. dave...