There are a lot of people (associated with parkour) that meet up at the art gallery on a Saturday morning. As you can see the group is very divided. Speak to the guys (and girl) about it. It wouldn't take much observation to see which guys are part of which group and they guys jumping about the most will be friendly enough. Alternatively I can arrange with them a meeting with you to discuss things?
Having Jumped around at the art gallery for 3 years and being good friends with the guys that still train there, I can tell you it's not jumps and landings dislodging the stone. All of the damage of the stonework has been done by vandals. If there are stones dislodged where the guys jump I am sure the damage was done/started by the vandals, jumping on the broken stonework might make matters worse but there's no way the damage was started by jumps and landings. However some (not all ...again vandals and corrosion) of the damage to the rails was the result of parkour, although I don't see where you're coming from with:
LEEDSGALL wrote:old folk and others complaining at how difficult it is to get up the steps
As far as I'm aware there are still railings on the side of the main steps and there is the disabled access.
Jumps first, internet discussion later... Much later.