Is there a way to randomise the way a destructible mesh collapses in each instance of it?
ive had a look and cant seem to find this anywhere and you cant access the seed for the mesh either
is this because the destruction has to be pre done and cant be done dynamically?
This is correct. You cannot change the settings in the Destructible Mesh Editor dynamically. These have to be setup before hand.
There is a card on the Unreal Engine 4 Roadmap to support dynamic destruction, but this probably will not be in the near future though before it would be investigated and implemented.