As the Sawyer says we dealt with the motorcycle problem by putting up signs saying "No Motorcycles or other motorised vehicles permitted" on all the entrances. The local dears know that if they now come on to our land they risk having their bikes confiscated and crushed as it is an offence ride a motorcycle on private land without permission (in our area the Police do confiscate and crush bikes). If you have not already done so I would report the quad bike visit to the police pointing out that the bikes are using a nature reserve as access, also have you contacted the owners or the reserve as you may be able to work together to stop the tresspass, also the site managers of the industrial estate may be of some help. As for dealing with the bikers if we meet them our method of threatening to dismantle their bike and fit it up their back passage works well, but I think we would have trouble fitting a quad bike up there :). More seriously there are usualy two of us with a van so we simply point out its private proprty so they risk the crushing of their bikes, we are also able to be about more than most as we work from the wood, though it is amazing what can be happening in one part while you are working in another.
John W