Check the fuse for the interior lights. If this fuse blows it will also cut out the central locking.

When I worked at my Seat dealer, we had identical problems on the Leon's (virtually identical under the skin) and it was always the fuse. Seat technical issued a bulletin to the dealers about the issue and advised to replace the 10 amp fuse for a 15 amp one. Due to the electrical demand (all the interior lights on and central locking being operated at the same time) on that circuit the 10 amp wasn't man enough for the job.

Hope that helps.