If you ring up gypsum they will say you should use their product to seal and key their finish. So if you as a last resort(And I would not advise if possible ) went down the legal route the question would be did he apply as per maufacturers recommendations? Answer - No.
Someone here will chip in with legalese from Consumer legislation, but try and work a compromise as he may have skimmed walls like this before with no problems and assumed the conditions were the same, I don't think it was deliberate, just ignorance or inexperience
Some might might run their business in that way ie I'm finished, what happens next is your problem, , and good luck to them - but if for example I plastered over a crack that ran down the middle of the wall without remedying it or advising that it might open again, then I am not doing the job correctly.
Talk to the man, see what he will do, it may be that he has some integrity and will make som,e effort to help a solution,
The cracks are not shrinkage or settlement, both of which you should make a client aware of before starting, but down to incorrect preparation.