Fixed.Few links may be of interest: http://www.cfa.org.uk/userfiles/files/Guidance Note - Plywood - v1 - July 2013.pdf
https://www.tiles.org.uk/wp-content...rnal-Ceramic-Tiling-Technical-Publication.pdf
I think you have a case against Weber. Contact your area rep and talk it through. Casually mention that it's a failure now being discussed openly on the uk's largest tile forum. Bad press - not good for business!
I can't link to the TTA technical paper - just google "tile association ply warning"
It's the first result...