Jacko,
On the face of it the shop clearly has a claim against the builders (who may then invoke their insurance) but make sure that you have clear authorisation to proceed, ideally from the shop themselves, with payment to you coming from the shop - what happens thereafter is not your concern.
The sooner the job is done the better as mastic has a habit of getting tracked around and becoming impacted.
There are many solvents that will tackle both Bitumen and mastic, smelly white spirit, Solvall, citrus-based products etc. but I would use Eco Spotter. Whatever you choose, do not apply liquid solvents direct to the carpet - dampen a cloth and apply with this.