In reply to petegunn:
I am Jack's complete lack of surprise.
As others have said, this issue has been going on for decades, the most recent saga I can remember was the bat boxes. I can also confirm that thick lube dabbed on all holds near the ground was a thing well into the 2000s.
My advice would be to make sure you are not flouting the rules of Phase 1: The Phantom Phase and other than that, ignore this latest attempt at intimidation and just get climbing. If the police are called then obviously respond politely but ultimately the burden for wasted police time lies with the land owner whose habit is to use any and all excuses and illegal methods they can to prevent climber access.