Don't think it's the decorators fault, or the OP who bought the paint, although either of those folk checking would've stopped it. Sounds likely that dulux knew the paint wasn't correct and I'd talk to dulux again before going to small claims. Didn't dulux have to pay out a lot of money when their oil-based paints started yellowing? They admitted their fault in that.
the yellowing paint was the result of the 2010 VOC compliance. Paint makers were directed to reduce the levels of solvents in their oil based paints. I am not sure that the Dulux paints were worse than other brands. I suspect that as the highest profile firm, they took the most flack.
The irony is that if a client tells me they want a non yellowing white paint, I am now more likely to use acid cat, which has much, much higher VOC levels than the old oil based paints