Within six months the onus is on the shop to disprove a fault, not on you to prove it.
The store is liable to rectify any issues (not Apple), but it’s up to them whether they decide to repair or replace. Someone ^^ said you can “reject the goods” and demand a refund; this is incorrect (you can only do that within a short time frame which allows you to inspect the goods).
Given that it’s a Mac, you may well be better served going direct to Apple rather than dealing with Curry’s, as their customer service is excellent. Bear in mind though that a per the previous paragraph it would be your choice if you did this, Curry’s cannot just pass the buck and tell you to go to Apple.