Every year, Morden Hall Park plays host to a fair, with various activities and entertainments. And every year, an area in the north of the park grounds is turned into a large car park. The vegetation is cut back and the doors are opened to anyone who wants to come by car (and I don't believe there's even a charge).
Morden Hall Park is very well served by public transport. Morden underground station is served by regular Northern Line services. The Croydon Tramlink has three stops within easy walking distance. Many buses run past the park grounds. South Merton station (mainline) is a 1km walk away. Almost uniquely in London, there's a decent off-road cycle route in the form of the Wandle Trail that leads straight to Morden Hall Park. There's also a car park for the Morden Hall garden centre. So why is it necessary for an organization like the National Trust, that claims to take protection of the environment seriously, to create a new car park, encouraging people to come by car to a place that is so well-served by other transport options?