It's a tricky one, but I think in that case 'The Reasonable Tiler' might refuse to do it (I'm not actually sure though without doing a 100 party survey!) so perhaps it's more fair to impose the duty.
Tiles falling off the wall is due to fixing that isn't safe (no matter who insists on it), so it's the quality of the job that causes the danger. With slippery tiles, it's the product that causes the risk, no matter how they are laid, ultimately it's the slippery factor that is dangerous.
I'm really not sure though; probably isn't a right answer. There are too many grey areas in Tort, which is one of the reasons I'm in this industry, rather than in the court room wearing a lovely set of robes! 😉
PTV of 36 minimum is the UK Law.... apparently. Not seen it enforced mind. So IF that WAS the case, does that change anything?