Have a look at the following thread on National Trust, its been done to death
Also I’d take the fine, its cheaper than their parking charges
Penalties for Infringement of Byelaws
26. Every person who shall offend against any, of the foregoing Byelaws shall be
liable on summary conviction to a fine not exceeding £20 and in the case of a
continuing offence to a further fine not exceeding £2 for each day during which the
offence continues,