It wasn't weed - we thought this too but checked it out futher and it wasn't weed it was something else. Not sure what but assumed it was something to do with the business which used to be run there (something to do with biochemicals IIRC)
The business was Reed Corrigated Card and then later under the SCA group. It actually closed down in 1994.
Security was never as bad. In 2007 you could just walk in. Only last 5-6 years has it been the random aggressive stuff you see now but I doubt it’s anything illegal as the police get called fairly often if you’re caught inside..