Good evening,
when laying sandstone onto cement the back of the slabs need to be primed, sandstone does not stick to cement like other types of paving. if you do not prime the slab first it will become looses from the cement. also by priming the back of the slabs this stops the cement coming through the slab and making them change colour.
you are right to not pay for the labour. because you have paid for the materials they are yours and the company cannot come and take or damage them. the whole lot of paving will need to come up and be done again with the sand stone primed first. you should be able to get the plans up whole so all i can say is that you don't pay the first company and use the money to pay another company to put right the issues. this will however have more cost to you.
i hope this help?