Just a point, no matter what you get in writing, you as the fixer are responsible for the job, if you use materials not suitable for the purpose and it fails, you are responsible.
Alan i beg to differ if the architects specify what materials are used then it is not the fixers problem if they use specified materials,all materials will be specified prior to installation and will be noted on drawings etc,hence its not the fixers problem if the tiles fail as long as they have gone to the architects spec,end of story.
Regardless of the what the fixer thinks the architect is to blame for not checking,if they fail the fixer then says i have used the adhesive etc that was specified and it will be in writing on all the elevation drawings on site,hence fixer is covered as long as they use specified materials