Hi Des511,
Welcome to the forums
Don't forget your grout will be a couple of kilos at least per m2 depending on the size of the grout line, so that would put you right on the limit of what the plasterboard can take.
Go around each wall and thump the walls with the palm of your hand and if you hear the sheet rattle against the studs then more screws will need to be inserted about 200mm apart this should give you some extra support for those lovely trav or limestone tiles you desire.
