Council owned green space is being used as a car park to avoid street traffic restrictions

The council owned 'triangle' of land down Stanley Hill Avenue has been used as a car park for the last 6 months. More recently the volume of cars and vans has increased and the wet weather has led to damage to the grass area. Commuters and motorists have been avoiding the parking restrictions in the street, by parking on this council owned land. I have lived in the street for 28 years and this land has never been used as a car park until recently. This morning 10 vehicles were parked on this small patch of land. Amersham Town Council say that it's a Bucks County Council issue. The problem has been reported twice on FixMyStreet under parking and immediately closed, yet not action has been taken. I spoke to Bucks County Council team responsible for streets today who explained that it isn't illegal to park in a council owned open space, unless there is a relevant by-law in place. Does this mean that any council owned public parks can be used as a car park? I was advised to re-report the problem to the Open Spaces team. Please can you assess whether the parking is legal or not and determine if a new by-law is required.