Contractor told me to get five bags of adhesive, no further info! Tiles are 20x10 7mm ceramic metro tiles. Roughly 3m2. Surface is skimmed and primed plasterboard. Mapei options i was considering: Rapid Set (20kg), Kerabond T or Keraquick. Not sure flexible adhesive is necessary, but i guess i only need one bag so price isn't really an issue. Any thoughts?