In reply to partz: You are no doubt talking about the Gromlech layby. Well only the local council or police can close the layby, but nobody is allowed to 'book it' exclusively. Unfortunately it is mostly filled by cars that don't want to pay their £10 to park at Peny Pass. So if climbers arrive late no spaces are available.
There is a regular bus that serves Pen y Pass for tourists and walkers so why can't they leave the laybys in the Pass for climbers. My personal view. The only advice I can offer is definitely not to park outside of the designated areas such as on double yellow lines, that line the Pass, as the police are very strict on this. You will be booked.