@Belfry prompted me to give the matter of parking some more thought so I actually read the email from the library. It says…
As your booking falls outside of our opening hours , please note that access to the underground parking area will be closed off approximately 30 minutes after the library has closed and there will be no access to any vehicles parked in this area after this time. Please park in the car park (adjacent to the pool) if you are travelling to the library by car.
…so I think we were lucky to find that cleaner to bail is out last time. 